Fix a bug in drawBuffers optimization.

gpu/command_buffer/service/framebuffer_manager.h

Index: gpu/command_buffer/service/framebuffer_manager.h
diff --git a/gpu/command_buffer/service/framebuffer_manager.h b/gpu/command_buffer/service/framebuffer_manager.h
index d2ebe070e5a30bf36d6189d89312c6d0cec72dc4..e4433cc045dbd3e0a342850a3d68facdac1f1f65 100644
--- a/gpu/command_buffer/service/framebuffer_manager.h
+++ b/gpu/command_buffer/service/framebuffer_manager.h
@@ -194,6 +194,11 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
   bool ValidateAndAdjustDrawBuffers(uint32_t fragment_output_type_mask,
                                     uint32_t fragment_output_written_mask);
 
+  // Filter out the draw buffers that have no images attached but are not NONE
+  // through DrawBuffers, to be on the safe side.
+  // This is applied before a clear call.
+  void AdjustDrawBuffers();
+
   bool ContainsActiveIntegerAttachments() const;
 
   // Return true if any draw buffers has an alpha channel.
@@ -247,6 +252,9 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
   // This call is only valid on a complete fbo.
   void UpdateDrawBufferMasks();
 
+  // Helper for ValidateAndAdjustDrawBuffers() and AdjustDrawBuffers().
+  void AdjustDrawBuffersImpl(uint32_t desired_mask);
+
   // The managers that owns this.
   FramebufferManager* manager_;