Implement native GMB backbuffers in the GLES2 Command Decoder.

components/mus/gles2/command_buffer_driver.cc

Index: components/mus/gles2/command_buffer_driver.cc
diff --git a/components/mus/gles2/command_buffer_driver.cc b/components/mus/gles2/command_buffer_driver.cc
index 7e0a06ecbd27c7fa66df8c720d1c9e3f2f031f57..b3821b9f4dec1acc7f5489955097dfc64cdf761a 100644
--- a/components/mus/gles2/command_buffer_driver.cc
+++ b/components/mus/gles2/command_buffer_driver.cc
@@ -130,13 +130,15 @@ bool CommandBufferDriver::Initialize(
   const bool bind_generates_resource = attrib_helper.bind_generates_resource;
   scoped_refptr<gpu::gles2::FeatureInfo> feature_info =
       new gpu::gles2::FeatureInfo(gpu_state_->gpu_driver_bug_workarounds());
+  // TODO(erikchen): The ContextGroup needs a reference to the
+  // GpuMemoryBufferManager.
   scoped_refptr<gpu::gles2::ContextGroup> context_group =
       new gpu::gles2::ContextGroup(
           gpu_state_->gpu_preferences(), gpu_state_->mailbox_manager(),
           new GpuMemoryTracker,
           new gpu::gles2::ShaderTranslatorCache(gpu_state_->gpu_preferences()),
           new gpu::gles2::FramebufferCompletenessCache, feature_info,
-          bind_generates_resource);
+          bind_generates_resource, nullptr);
 
   command_buffer_.reset(
       new gpu::CommandBufferService(context_group->transfer_buffer_manager()));