Implement EXT_draw_buffers WebGL extention support in command buffer.

gpu/command_buffer/service/framebuffer_manager.h

Index: gpu/command_buffer/service/framebuffer_manager.h
===================================================================
--- gpu/command_buffer/service/framebuffer_manager.h	(revision 187725)
+++ gpu/command_buffer/service/framebuffer_manager.h	(working copy)
@@ -114,6 +114,10 @@
   // Check all attachments are cleared
   bool IsCleared() const;
 
+  GLenum GetDrawBuffer(GLenum draw_buffer) const;
+
+  void SetDrawBuffers(GLsizei n, const GLenum* bufs);
+
   static void ClearFramebufferCompleteComboMap();
 
  private:
@@ -160,6 +164,8 @@
   typedef base::hash_map<std::string, bool> FramebufferComboCompleteMap;
   static FramebufferComboCompleteMap* framebuffer_combo_complete_map_;
 
+  scoped_array<GLenum> draw_buffers_;
+
   DISALLOW_COPY_AND_ASSIGN(Framebuffer);
 };
 
@@ -167,7 +173,7 @@
 // so we can correctly clear them.
 class GPU_EXPORT FramebufferManager {
  public:
-  FramebufferManager();
+  explicit FramebufferManager(uint32 max_draw_buffers);
   ~FramebufferManager();
 
   // Must call before destruction.
@@ -221,6 +227,8 @@
 
   bool have_context_;
 
+  uint32 max_draw_buffers_;
+
   DISALLOW_COPY_AND_ASSIGN(FramebufferManager);
 };