Don't enter gesture event handling when a frame is detached.

Some code paths are unguarded for the case where we get events while shutting
down/detaching a Frame. With this patch we just early-out.

BUG=655306

Review-Url: https://codereview.chromium.org/2540573002
Cr-Commit-Position: refs/heads/master@{#434876}

third_party/WebKit/Source/core/input/EventHandler.cpp

@@ -1350,6 +1350,8 @@ WebInputEventResult EventHandler::handleGestureEvent(
 WebInputEventResult EventHandler::handleGestureEvent(
     const GestureEventWithHitTestResults& targetedEvent) {
   TRACE_EVENT0("input", "EventHandler::handleGestureEvent");
+  if (!m_frame->host())
+    return WebInputEventResult::NotHandled;
 
   // Propagation to inner frames is handled below this function.
   ASSERT(m_frame == m_frame->localFrameRoot());
@@ -1381,12 +1383,17 @@ WebInputEventResult EventHandler::handleGestureEventInFrame(
 WebInputEventResult EventHandler::handleGestureScrollEvent(
     const PlatformGestureEvent& gestureEvent) {
   TRACE_EVENT0("input", "EventHandler::handleGestureScrollEvent");
+  if (!m_frame->host())
+    return WebInputEventResult::NotHandled;
 
   return m_scrollManager->handleGestureScrollEvent(gestureEvent);
 }
 
 WebInputEventResult EventHandler::handleGestureScrollEnd(
     const PlatformGestureEvent& gestureEvent) {
+  if (!m_frame->host())
+    return WebInputEventResult::NotHandled;
+
   return m_scrollManager->handleGestureScrollEnd(gestureEvent);
 }