Plumb whether or not a frame is in shadow DOM to the embedder.

Source/core/frame/FrameClient.h

Index: Source/core/frame/FrameClient.h
diff --git a/Source/core/frame/FrameClient.h b/Source/core/frame/FrameClient.h
index 06e7ebe97f65900f7ac55ee26722f819f64bce39..70e5d959f52a689721518d99ad9f27fdef0ac34c 100644
--- a/Source/core/frame/FrameClient.h
+++ b/Source/core/frame/FrameClient.h
@@ -16,6 +16,10 @@ class SecurityOrigin;
 
 class CORE_EXPORT FrameClient {
 public:
+    virtual bool inShadowTree() const = 0;
+
+    // TODO(dcheng): Move this into FrameLoaderClient, since remote frames don't
+    // need this.
     virtual void willBeDetached() = 0;
     virtual void detached() = 0;