Bug 1859625 Part 4: Do feature support for OPENGL_COMPOSITING in InitOpenGLConfig. r=jgilbert

This feature needs to have defaults set before user force enables are
processed. Before this change, both ImportContentDeviceData and
InitOpenGLConfig potentially could do both defaults and user setting,
making it an error if both functions are called (in either order). This
change makes it so that InitOpenGLConfig is solely responsible for
setting defaults and user force enable for this feature. This change
prevents the content process from inheriting parent process data
available in ImportContentDeviceData, but as long as InitOpenGLConfig
fully configures the feature and makes the right decision, that doesn't
matter.

Differential Revision: https://phabricator.services.mozilla.com/D194436

gfx/thebes/gfxPlatform.cpp

@@ -3892,7 +3892,10 @@ void gfxPlatform::ImportContentDeviceData(
 
   const DevicePrefs& prefs = aData.prefs();
   gfxConfig::Inherit(Feature::HW_COMPOSITING, prefs.hwCompositing());
-  gfxConfig::Inherit(Feature::OPENGL_COMPOSITING, prefs.oglCompositing());
+
+  // We don't inherit Feature::OPENGL_COMPOSITING here, because platforms
+  // will handle that (without imported data from the parent) in
+  // InitOpenGLConfig.
 
   gCMSOutputProfileData = Some(aData.cmsOutputProfileData().Clone());
 }