PlzNavigate: Allow frames to fallback to alternative content.

third_party/WebKit/Source/core/frame/FrameOwner.h

Index: third_party/WebKit/Source/core/frame/FrameOwner.h
diff --git a/third_party/WebKit/Source/core/frame/FrameOwner.h b/third_party/WebKit/Source/core/frame/FrameOwner.h
index 23f716a89ee317b4fab3298f9f39563e5d25a75a..d1d529286bb3475f545f0ac60f6b39bebf9b7613 100644
--- a/third_party/WebKit/Source/core/frame/FrameOwner.h
+++ b/third_party/WebKit/Source/core/frame/FrameOwner.h
@@ -35,6 +35,7 @@ class CORE_EXPORT FrameOwner : public GarbageCollectedMixin {
 
   // On load failure, a frame can ask its owner to render fallback content
   // which replaces the frame contents.
+  virtual bool canRenderFallbackContent() const = 0;
   virtual void renderFallbackContent() = 0;
 
   virtual ScrollbarMode scrollingMode() const = 0;
@@ -60,6 +61,7 @@ class CORE_EXPORT DummyFrameOwner
   void clearContentFrame() override {}
   SandboxFlags getSandboxFlags() const override { return SandboxNone; }
   void dispatchLoad() override {}
+  bool canRenderFallbackContent() const override { return false; }
   void renderFallbackContent() override {}
   ScrollbarMode scrollingMode() const override { return ScrollbarAuto; }
   int marginWidth() const override { return -1; }