Fix crash in BlitFramebufferCHROMIUM if a read or draw depth/stencil buffer is not present

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 b5de696c4c1394db806c3dda79823f3256dae131..e5962f7236da53e6373ea1a49269addc3f781d85 100644
--- a/gpu/command_buffer/service/framebuffer_manager.h
+++ b/gpu/command_buffer/service/framebuffer_manager.h
@@ -151,6 +151,12 @@ class GPU_EXPORT Framebuffer : public base::RefCounted<Framebuffer> {
   bool HasColorAttachment(int index) const;
   bool HasDepthAttachment() const;
   bool HasStencilAttachment() const;
+  // The following look at DEPTH_STENCIL_ATTACHMENT or
+  // DEPTH_ATTACHMENT/STENCIL_ATTACHMENT.
+  // TODO(piman): DEPTH_STENCIL_ATTACHMENT is not a real attachment point,
+  // except for WebGL1. Fix this.  http://crbug.com/630568
+  const Attachment* GetDepthAttachment() const;
+  const Attachment* GetStencilAttachment() const;
   GLenum GetDepthFormat() const;
   GLenum GetStencilFormat() const;
   GLenum GetDrawBufferInternalFormat() const;