Enable FeatureList for the GPU process.

content/common/child_process_host_impl.h

Index: content/common/child_process_host_impl.h
diff --git a/content/common/child_process_host_impl.h b/content/common/child_process_host_impl.h
index 18875c94e67028db164d96d82c55bb1e7d74dfc8..0c74fd7f125e8ce62e4f6f70f9f76c9bd599745a 100644
--- a/content/common/child_process_host_impl.h
+++ b/content/common/child_process_host_impl.h
@@ -23,6 +23,7 @@
 #include "ui/gfx/gpu_memory_buffer.h"
 
 namespace base {
+class CommandLine;
 class FilePath;
 }
 
@@ -61,6 +62,11 @@ class CONTENT_EXPORT ChildProcessHostImpl : public ChildProcessHost,
   // This will never return ChildProcessHost::kInvalidUniqueID.
   static int GenerateChildProcessUniqueId();
 
+  // Copies kEnableFeatures and kDisableFeatures to the command line. Generates
+  // them from the FeatureList override state, to take into account overrides
+  // from FieldTrials.
+  static void CopyFeatureAndFieldTrialFlags(base::CommandLine* cmd_line);

[jam, 2016/05/03 19:45:08]

this code is only called from content/browser, so it doesn't below in
content/common (i.e. code shared between browser and child processes). you can
add this static method to BrowserChildProcessHostImpl or create new files for it
in content/browser

[erikchen, 2016/05/03 19:55:05]

I added a static method to BrowserChildProcessHostImpl.

+
   // Derives a tracing process id from a child process id. Child process ids
   // cannot be used directly in child process for tracing due to security
   // reasons (see: discussion in crrev.com/1173263004). This method is meant to