Disable FRAMEBUFFER_SRGB when restoring from a null context state

gpu/command_buffer/service/context_state.h

Index: gpu/command_buffer/service/context_state.h
diff --git a/gpu/command_buffer/service/context_state.h b/gpu/command_buffer/service/context_state.h
index 2378eeb3906693caec6f0b98b451a2b5eeedc1c9..7c53a72ba78dc23acdac519c8974dfb204e9a0a1 100644
--- a/gpu/command_buffer/service/context_state.h
+++ b/gpu/command_buffer/service/context_state.h
@@ -153,6 +153,12 @@ GPU_EXPORT void Vec4::SetValues<GLint>(const GLint* values);
 template <>
 GPU_EXPORT void Vec4::SetValues<GLuint>(const GLuint* values);
 
+enum FramebufferSRGBState {
+  FRAMEBUFFER_SRGB_UNDEFINED,

[Zhenyao Mo, 2016/08/15 22:37:27]

Do we really need this?  If the prev_context is null, it means it's the context
initialization, which means FRAMEBUFFER_SRGB is disabled.

[qiankun, 2016/08/15 23:01:06]

Yes, we do need this. See the bug this CL will fix:
https://bugs.chromium.org/p/chromium/issues/detail?id=637801. 

In context initialization, we should call glDisable(FRAMEBUFFER_SRGB) to do
disable because before this initialization other context may enable
FRAMEBUFFER_SRGB and EnableDisableFramebufferSRGB(false) will not do real
disable as I pointed in the description of this CL.

[Zhenyao Mo, 2016/08/16 17:16:48]

This might fix the bug, but it's not correct.  If RestoreState() is called with
previous_state set to nullptr, the FramebufferSRGB isn't undefined.  It should
be initialized to false.  In this situation, you should just set it to disabled,
and call glDisable(FRAMEBUFFER_SRGB) to make sure of it.

[qiankun, 2016/08/16 21:30:09]

framebuffer_srgb_ is used to avoid redundant glEnable/glDisable call. In the
case you pointed,  setting framebuffer_srg b_ as FRAMEBUFFER_SRGB_UNDEFINED will
make the following EnableDisableFramebufferSRGB() do the real glEnable/glDisable
at render time. 

[qiankun, 2016/08/16 21:42:46]

FRAMEBUFFER_SRGB is set dynamically on demand in current implementation. So it
doesn't matter if it's enabled or disabled at context initialization time. We
will set it properly according the content.

+  FRAMEBUFFER_SRGB_DISABLED,
+  FRAMEBUFFER_SRGB_ENABLED
+};
+
 struct GPU_EXPORT ContextState {
   enum Dimension {
     k2D,
@@ -272,7 +278,7 @@ struct GPU_EXPORT ContextState {
   PixelStoreParams GetPackParams();
   PixelStoreParams GetUnpackParams(Dimension dimension);
 
-  void EnableDisableFramebufferSRGB(bool enable);
+  void EnableDisableFramebufferSRGB(FramebufferSRGBState enable);
 
   #include "gpu/command_buffer/service/context_state_autogen.h"
 
@@ -341,7 +347,7 @@ struct GPU_EXPORT ContextState {
 
   void InitStateManual(const ContextState* prev_state) const;
 
-  bool framebuffer_srgb_;
+  FramebufferSRGBState framebuffer_srgb_;
 
   // Generic vertex attrib base types: FLOAT, INT, or UINT.
   // Each base type is encoded into 2 bits, the lowest 2 bits for location 0,