Framebuffer clear() needs to consider the situation some draw buffers are disabled.

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 129d5f348ef65de63410a7cdf0635c0b872f4203..b68ab7d9f1209710bc994d02bc1ebb7c9344e2a5 100644
--- a/gpu/command_buffer/service/framebuffer_manager.h
+++ b/gpu/command_buffer/service/framebuffer_manager.h
@@ -64,6 +64,7 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
   }
 
   bool HasUnclearedAttachment(GLenum attachment) const;
+  bool HasUnclearedColorAttachments() const;
 
   void MarkAttachmentAsCleared(
     RenderbufferManager* renderbuffer_manager,
@@ -129,6 +130,14 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
 
   void SetDrawBuffers(GLsizei n, const GLenum* bufs);
 
+  // If a color buffer is attached to GL_COLOR_ATTACHMENTi, enable that
+  // draw buffer for glClear().
+  void PrepareDrawBuffersForClear() const;
+
+  // Restore draw buffers states that have been changed in
+  // PrepareDrawBuffersForClear().
+  void RestoreDrawBuffersAfterClear() const;
+
   // Return true if any draw buffers has an alpha channel.
   bool HasAlphaMRT() const;
 
@@ -163,6 +172,10 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
     return framebuffer_complete_state_count_id_;
   }
 
+  // Helper function for PrepareDrawBuffersForClear() and
+  // RestoreDrawBuffersAfterClear().
+  void ChangeDrawBuffersHelper(bool recover) const;
+
   // The managers that owns this.
   FramebufferManager* manager_;