adb: add dry-run option to push/sync.

Make it easier to benchmark file sync performance by ignoring the file
system.

Bug: https://issuetracker.google.com/150827486
Test: test_device.py
Change-Id: Icfa4b28eb5206f1914c0c163833d070a3748c3ea
diff --git a/file_sync_protocol.h b/file_sync_protocol.h
index 90bd763..8f8f85f 100644
--- a/file_sync_protocol.h
+++ b/file_sync_protocol.h
@@ -93,6 +93,7 @@
     kSyncFlagNone = 0,
     kSyncFlagBrotli = 1,
     kSyncFlagLZ4 = 2,
+    kSyncFlagDryRun = 0x8000'0000U,
 };
 
 enum class CompressionType {