Fix crash in WasTargetForLastMouseEvent

The frame could be null, ensure if it is we return early.

BUG=1130991

Change-Id: I8ced1223f4beb281adcb06c99d9b0fc4b761333e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2468237Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816763}

third_party/blink/renderer/core/exported/web_plugin_container_impl.cc

@@ -673,10 +673,10 @@ gfx::Point WebPluginContainerImpl::LocalToRootFramePoint(
 }
 
 bool WebPluginContainerImpl::WasTargetForLastMouseEvent() {
-  return element_->GetDocument()
-             .GetFrame()
-             ->GetEventHandler()
-             .GetElementUnderMouse() == element_;
+  auto* frame = element_->GetDocument().GetFrame();
+  if (!frame)
+    return false;
+  return frame->GetEventHandler().GetElementUnderMouse() == element_;
 }
 
 void WebPluginContainerImpl::DidReceiveResponse(