Add back early return for MainFrameImpl being null.

In https://crrev.com/4bb0db09 I removed an early return for
MainFrameImpl being null and added a CHECK.

It is clear that CHECK is being hit (we've seen this in
EndCompositorFrame as well with issue 1139104) and I speculate that
the LocalRootImpl is non-null but MainFrameImpl is null.

The only code that still accesses the MainFrameImpl is this piece of
HandleGestureEvent as it hasn't been converted yet. Since branch for
M88 is coming up silence this condition that was previously an early
return.

BUG=1148018,1146579

Change-Id: I1d214c8787fcd8e4dcfe5983752b3468373f7321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532990
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826875}

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

@@ -313,6 +313,12 @@ WebInputEventResult WebViewFrameWidget::HandleGestureEvent(
     return WebInputEventResult::kNotHandled;
   }
 
+  // TODO(https://crbug.com/1148346): We need to figure out why MainFrameImpl is
+  // null but LocalRootImpl isn't.
+  CHECK(LocalRootImpl());
+  if (!web_view_->MainFrameImpl())
+    return WebInputEventResult::kNotHandled;
+
   WebInputEventResult event_result = WebInputEventResult::kNotHandled;
   bool event_cancelled = false;  // for disambiguation