Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

gpu/GLES2/gl2extchromium.h

Index: gpu/GLES2/gl2extchromium.h
diff --git a/gpu/GLES2/gl2extchromium.h b/gpu/GLES2/gl2extchromium.h
index 584d29a267d96737a951a9d45682eb61bd6da913..9e464c7686cfb0c693c9f2f8e9b093daefd16724 100644
--- a/gpu/GLES2/gl2extchromium.h
+++ b/gpu/GLES2/gl2extchromium.h
@@ -520,6 +520,16 @@ typedef GLboolean (GL_APIENTRYP PFNGLENABLEFEATURECHROMIUMPROC) (
 #endif
 #endif  /* GL_CHROMIUM_command_buffer_latency_query */
 
+/* GL_CHROMIUM_screen_space_antialiasing */
+#ifndef GL_CHROMIUM_screen_space_antialiasing
+#define GL_CHROMIUM_screen_space_antialiasing 1
+#ifdef GL_GLEXT_PROTOTYPES
+GL_APICALL void GL_APIENTRY glApplyScreenSpaceAntialiasingCHROMIUM();
+#endif
+typedef void (GL_APIENTRYP PFNGLAPPLYSCREENSPACEANTIALIASINGCHROMIUMPROC) ();
+#endif /* GL_CHROMIUM_screen_space_antialiasing */
+
+
 /* GL_ARB_robustness */
 #ifndef GL_ARB_robustness
 #define GL_ARB_robustness 1