Fix crash after HandleMouseUp.

Looking at crash reports where the LocalRootImpl is null it is clear
that the HandleMouseUp can detach the frame and the GetPage accessor
will then be null.

Change-Id: If856e914135b833bfacbce84be5c15b242acd09f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533021
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826381}

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

@@ -546,6 +546,9 @@ void WebFrameWidgetBase::HandleMouseDown(LocalFrame& main_frame,
   }
 
   PageWidgetEventHandler::HandleMouseDown(main_frame, event);
+  // PageWidgetEventHandler may have detached the frame.
+  if (!LocalRootImpl())
+    return;
 
   if (view_impl->GetPagePopup() && page_popup &&
       view_impl->GetPagePopup()->HasSamePopupClient(page_popup.get())) {
@@ -606,6 +609,9 @@ WebInputEventResult WebFrameWidgetBase::HandleMouseUp(
     const WebMouseEvent& event) {
   WebInputEventResult result =
       PageWidgetEventHandler::HandleMouseUp(main_frame, event);
+  // PageWidgetEventHandler may have detached the frame.
+  if (!LocalRootImpl())
+    return result;
 
   if (GetPage()->GetSettings().GetShowContextMenuOnMouseUp()) {
     // Dispatch the contextmenu event regardless of if the click was swallowed.
@@ -621,6 +627,7 @@ WebInputEventResult WebFrameWidgetBase::HandleMouseWheel(
     const WebMouseWheelEvent& event) {
   View()->CancelPagePopup();
   return PageWidgetEventHandler::HandleMouseWheel(frame, event);
+  // PageWidgetEventHandler may have detached the frame.
 }
 
 WebInputEventResult WebFrameWidgetBase::HandleCharEvent(