Enable FeatureList for the GPU process.

content/common/child_process_host_impl.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_COMMON_CHILD_PROCESS_HOST_IMPL_H_
 #define CONTENT_COMMON_CHILD_PROCESS_HOST_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/memory/shared_memory.h"
 #include "base/memory/singleton.h"
 #include "base/process/process.h"
 #include "base/strings/string16.h"
 #include "build/build_config.h"
 #include "content/public/common/child_process_host.h"
 #include "ipc/ipc_listener.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 
 namespace base {
+class CommandLine;
 class FilePath;
 }
 
 namespace IPC {
 class MessageFilter;
 }
 
 namespace gpu {
 struct SyncToken;
 }
@@ skipping 18 matching lines @@
   // function will be used to generate the id_, but it is also used to generate
   // IDs for the RenderProcessHost, which doesn't inherit from us, and whose IDs
   // must be unique for all child processes.
   //
   // This function is threadsafe since RenderProcessHost is on the UI thread,
   // but normally this will be used on the IO thread.
   //
   // 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
   // be used when tracing for identifying cross-process shared memory from a
   // process which knows the child process id of its endpoints. The value
   // returned by this method is guaranteed to be equal to the value returned by
   // MemoryDumpManager::GetTracingProcessId() in the corresponding child
   // process.
   //
   // Never returns MemoryDumpManager::kInvalidTracingProcessId.
@@ skipping 45 matching lines @@
   // thread, we don't have a IPC::ChannelProxy and so we manage filters
   // manually.
   std::vector<scoped_refptr<IPC::MessageFilter> > filters_;
 
   DISALLOW_COPY_AND_ASSIGN(ChildProcessHostImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_CHILD_PROCESS_HOST_IMPL_H_