update_engine: Split payload signing and verification.

Payloads are only signed on server-side code (delta_generator) and
verified on both sides and unittest. This removes the dependency of
payload_generator/ code from delta_performer.cc by spliting the
payload signing and verification in two files.

Currently, both files are still included on all the built files.

This patch also includes some minor linter fixes.

BUG=chromium:394184
TEST=FEATURES="test" emerge-link update_engine; sudo emerge update_engine

Change-Id: Ia4268257f4260902bc37612f429f44ba7e8f65fd
Reviewed-on: https://chromium-review.googlesource.com/208540
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/update_engine.gyp b/update_engine.gyp
index 24a9a98..c201b65 100644
--- a/update_engine.gyp
+++ b/update_engine.gyp
@@ -164,10 +164,11 @@
         'payload_generator/full_update_generator.cc',
         'payload_generator/graph_utils.cc',
         'payload_generator/metadata.cc',
+        'payload_generator/payload_signer.cc',
         'payload_generator/tarjan.cc',
         'payload_generator/topological_sort.cc',
-        'payload_signer.cc',
         'payload_state.cc',
+        'payload_verifier.cc',
         'postinstall_runner_action.cc',
         'prefs.cc',
         'proxy_resolver.cc',
@@ -293,9 +294,9 @@
             'payload_generator/full_update_generator_unittest.cc',
             'payload_generator/graph_utils_unittest.cc',
             'payload_generator/metadata_unittest.cc',
+            'payload_generator/payload_signer_unittest.cc',
             'payload_generator/tarjan_unittest.cc',
             'payload_generator/topological_sort_unittest.cc',
-            'payload_signer_unittest.cc',
             'payload_state_unittest.cc',
             'postinstall_runner_action_unittest.cc',
             'prefs_unittest.cc',