WebGL: optimize to draw WebGL on external destination. e.g. 2D Canvas.

Source/platform/graphics/gpu/DrawingBuffer.h

Index: Source/platform/graphics/gpu/DrawingBuffer.h
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.h b/Source/platform/graphics/gpu/DrawingBuffer.h
index e60f25509dda6c436356a1f7070e07e342e0ef82..698b4a38a52941fbd3f45a4505e4508bce35beb5 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.h
+++ b/Source/platform/graphics/gpu/DrawingBuffer.h
@@ -246,7 +246,11 @@ private:
     Platform3DObject m_fbo;
     // DrawingBuffer's output is double-buffered. m_colorBuffer is the back buffer.
     TextureInfo m_colorBuffer;
-    TextureInfo m_frontColorBuffer;
+    struct FrontBufferInfo {
+        TextureInfo texInfo;
+        WebExternalTextureMailbox mailbox;
+    };
+    FrontBufferInfo m_frontColorBuffer;
 
     // This is used when we have OES_packed_depth_stencil.
     Platform3DObject m_depthStencilBuffer;