Add IPC allowing GPU process to tell browser process to temporarily drop a front buffer.

content/common/gpu/image_transport_surface.cc

Index: content/common/gpu/image_transport_surface.cc
===================================================================
--- content/common/gpu/image_transport_surface.cc	(revision 119674)
+++ content/common/gpu/image_transport_surface.cc	(working copy)
@@ -172,6 +172,10 @@
   return decoder->MakeCurrent();
 }
 
+void ImageTransportHelper::Suspend() {
+  manager_->Send(new GpuHostMsg_AcceleratedSurfaceSuspend(stub_->surface_id()));
+}
+
 gpu::GpuScheduler* ImageTransportHelper::Scheduler() {
   if (!stub_.get())
     return NULL;