Prevent kGestureDoubleTap from propagating for OOPIFs.

kGestureDoubleTap is used by Android only, and page scale animations
should only be iniitated in the main frame renderer. WebViewImpl has
special handling logic that avoids sending this event to EventHandler,
but the current (incomplete) implementation for OOPIFs in
WebFrameWidgetImpl does forward it to EventHandler, hitting a variety
of NOTREACHED points along the way.

This CL modifies WebFrameWidgetImpl to drop this event without sending
it to EventHandler, and return the same status as WebViewImpl does.

Bug: 734209
Change-Id: I900111df370a70b1db418660135e7e00e0f2508e
Reviewed-on: https://chromium-review.googlesource.com/1183942
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584990}

third_party/blink/renderer/core/frame/web_frame_widget_impl.cc

@@ -879,7 +879,16 @@ WebInputEventResult WebFrameWidgetImpl::HandleGestureEvent(
       View()->SetLastHiddenPagePopup(nullptr);
       break;
     case WebInputEvent::kGestureShowPress:
+      break;
     case WebInputEvent::kGestureDoubleTap:
+      // Until https://crbug.com/734209 is resolved and OOPIFs learn how to
+      // handle AnimateDoubleTap, we shouldn't pass this event to the event
+      // handler as it will just result in (at best) hitting NOTREACHED() in
+      // debug builds.
+      LOG(INFO) << "DoubleTap zoom animations not yet implemented for OOPIF.";
+      event_result = WebInputEventResult::kHandledSystem;
+      Client()->DidHandleGestureEvent(event, event_cancelled);
+      return event_result;
       break;
     case WebInputEvent::kGestureTwoFingerTap:
     case WebInputEvent::kGestureLongPress: