Add GL_CHROMIUM_screen_space_antialiasing to support alternative AA

public/platform/WebGraphicsContext3D.h

Index: public/platform/WebGraphicsContext3D.h
diff --git a/public/platform/WebGraphicsContext3D.h b/public/platform/WebGraphicsContext3D.h
index 0ecbba3f9dbd8eb976c5bbcfb7f9151f2a15ccd9..4960358ea020a7ffed47164c8996b8c2920ebf41 100644
--- a/public/platform/WebGraphicsContext3D.h
+++ b/public/platform/WebGraphicsContext3D.h
@@ -376,6 +376,9 @@ public:
 
     virtual WebString getTranslatedShaderSourceANGLE(WebGLId shader) = 0;
 
+    // GL_CHROMIUM_screen_space_antialiasing
+    virtual void applyScreenSpaceAntialiasingCHROMIUM() = 0;

[Ken Russell (switch to Gerrit), 2015/09/11 18:39:16]

This will either need to be given an empty implementation or the tests will need
to be updated. Probably adding an empty implementation is the easiest way to go.

+
     // GL_CHROMIUM_iosurface
     virtual void texImageIOSurface2DCHROMIUM(WGC3Denum target, WGC3Dint width, WGC3Dint height, WGC3Duint ioSurfaceId, WGC3Duint plane) { }