Fix nullptr crash in RootScrollerController

The stacktrace in the bug revealed a path where we don't check that the
LocalFrameView isn't null.

No test since I couldn't reproduce the crash.

Bug: 805420
Change-Id: Ief57e784e765efadb18b5c9c040e7b5a40ee904c
Reviewed-on: https://chromium-review.googlesource.com/887826Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532412}

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.cpp

@@ -252,6 +252,9 @@ void RootScrollerController::UpdateIFrameGeometryAndLayoutSize(
   LocalFrameView* child_view =
       ToLocalFrameView(frame_owner.OwnedEmbeddedContentView());
 
+  if (!child_view)
+    return;
+
   // We can get here as a result of the "post layout resize" on the main frame.
   // That happens from inside LocalFrameView::PerformLayout. Calling
   // UpdateGeometry on the iframe causes it to layout which calls