Hold the Pepper proxy lock for all Pepper GL calls.

ppapi/proxy/ppapi_command_buffer_proxy.cc

Index: ppapi/proxy/ppapi_command_buffer_proxy.cc
diff --git a/ppapi/proxy/ppapi_command_buffer_proxy.cc b/ppapi/proxy/ppapi_command_buffer_proxy.cc
index 7a2783a20064415baa55dc52cf01cfcc2a25b07d..86aa3c741bb3e69525a5ce083a336b47854790ed 100644
--- a/ppapi/proxy/ppapi_command_buffer_proxy.cc
+++ b/ppapi/proxy/ppapi_command_buffer_proxy.cc
@@ -219,7 +219,10 @@ int32 PpapiCommandBufferProxy::CreateGpuMemoryBufferImage(
 bool PpapiCommandBufferProxy::Send(IPC::Message* msg) {
   DCHECK(last_state_.error == gpu::error::kNoError);
 
-  if (channel_->Send(msg))
+  // We need hold the Pepper proxy lock for sync IPC, because GPU command buffer
+  // may use a sync IPC with another lock held. It may cause deadlock.
+  // http://crbug.com/418651
+  if (channel_->SendAndStayLocked(msg))
     return true;
 
   last_state_.error = gpu::error::kLostContext;