Do not share a GL context between CoreAnimation and async readback

content/browser/renderer_host/compositing_iosurface_context_mac.h

Index: content/browser/renderer_host/compositing_iosurface_context_mac.h
diff --git a/content/browser/renderer_host/compositing_iosurface_context_mac.h b/content/browser/renderer_host/compositing_iosurface_context_mac.h
index 357925b719b1601d566d0cd98a437b1b7d1b9c0f..f881dc1f831058915d4ac705acf7b0a534247a45 100644
--- a/content/browser/renderer_host/compositing_iosurface_context_mac.h
+++ b/content/browser/renderer_host/compositing_iosurface_context_mac.h
@@ -30,7 +30,13 @@ class CompositingIOSurfaceShaderPrograms;
 class CompositingIOSurfaceContext
     : public base::RefCounted<CompositingIOSurfaceContext> {
  public:
-  enum { kOffscreenContextWindowNumber = -2 };
+  enum {
+    // The number used to look up the context used for async readback and for
+    // initializing the IOSurface.
+    kOffscreenContextWindowNumber = -2,
+    // The number used to look up the context used by CAOpenGLLayers.
+    kCALayerContextWindowNumber = -3,
+  };
 
   // Get or create a GL context for the specified window with the specified
   // surface ordering. Share these GL contexts as much as possible because