Revert "Revert "Stop using the runtime classpath in dex2oat""

This reverts commit 053a84b762536144d5cae309ca5abd97766b4583.

Reason for revert: Prepare the CL again to be submitted after
https://chrome-internal-review.googlesource.com/c/423809 gets merged.

Change-Id: I1d0e3d6bf2aa16249b2d0146acd9c81149924c00
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 0826fa1..fd22dcd 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -1304,7 +1304,7 @@
       } else if (option.starts_with("--class-loader-context=")) {
         class_loader_context_ = ClassLoaderContext::Create(
             option.substr(strlen("--class-loader-context=")).data());
-        if (class_loader_context_== nullptr) {
+        if (class_loader_context_ == nullptr) {
           Usage("Option --class-loader-context has an incorrect format: %s", option.data());
         }
       } else if (!compiler_options_->ParseCompilerOption(option, Usage)) {
@@ -1569,20 +1569,12 @@
       }
 
       // Open dex files for class path.
+
       if (class_loader_context_ == nullptr) {
-        // TODO(calin): Temporary workaround while we transition to use
-        // --class-loader-context instead of --runtime-arg -cp
-        if (runtime_->GetClassPathString().empty()) {
-          class_loader_context_ = std::unique_ptr<ClassLoaderContext>(
-              new ClassLoaderContext());
-        } else {
-          std::string spec = runtime_->GetClassPathString() == OatFile::kSpecialSharedLibrary
-              ? OatFile::kSpecialSharedLibrary
-              : "PCL[" + runtime_->GetClassPathString() + "]";
-          class_loader_context_ = ClassLoaderContext::Create(spec);
-        }
+        // If no context was specified use the default one (which is an empty PathClassLoader).
+        class_loader_context_ = std::unique_ptr<ClassLoaderContext>(ClassLoaderContext::Default());
       }
-      CHECK(class_loader_context_ != nullptr);
+
       DCHECK_EQ(oat_writers_.size(), 1u);
 
       // Note: Ideally we would reject context where the source dex files are also
diff --git a/runtime/class_loader_context.cc b/runtime/class_loader_context.cc
index b50aec0..e7051b3 100644
--- a/runtime/class_loader_context.cc
+++ b/runtime/class_loader_context.cc
@@ -68,6 +68,10 @@
   }
 }
 
+std::unique_ptr<ClassLoaderContext> ClassLoaderContext::Default() {
+  return Create("");
+}
+
 std::unique_ptr<ClassLoaderContext> ClassLoaderContext::Create(const std::string& spec) {
   std::unique_ptr<ClassLoaderContext> result(new ClassLoaderContext());
   if (result->Parse(spec)) {
diff --git a/runtime/class_loader_context.h b/runtime/class_loader_context.h
index 85299d4..9afa880 100644
--- a/runtime/class_loader_context.h
+++ b/runtime/class_loader_context.h
@@ -34,9 +34,6 @@
 // Utility class which holds the class loader context used during compilation/verification.
 class ClassLoaderContext {
  public:
-  // Creates an empty context (with no class loaders).
-  ClassLoaderContext();
-
   ~ClassLoaderContext();
 
   // Opens requested class path files and appends them to ClassLoaderInfo::opened_dex_files.
@@ -126,6 +123,10 @@
   static std::unique_ptr<ClassLoaderContext> CreateContextForClassLoader(jobject class_loader,
                                                                          jobjectArray dex_elements);
 
+  // Returns the default class loader context to be used when none is specified.
+  // This will return a context with a single and empty PathClassLoader.
+  static std::unique_ptr<ClassLoaderContext> Default();
+
  private:
   enum ClassLoaderType {
     kInvalidClassLoader = 0,
@@ -151,6 +152,9 @@
     explicit ClassLoaderInfo(ClassLoaderType cl_type) : type(cl_type) {}
   };
 
+  // Creates an empty context (with no class loaders).
+  ClassLoaderContext();
+
   // Constructs an empty context.
   // `owns_the_dex_files` specifies whether or not the context will own the opened dex files
   // present in the class loader chain. If `owns_the_dex_files` is true then OpenDexFiles cannot