VR: Add null check when delivering UI input.

This is a speculative fix for FYI bot crashes.

Change-Id: I8cea6aa917eb32c93f8a0f1967eb630d8c2fca88
Reviewed-on: https://chromium-review.googlesource.com/881663Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531315}

chrome/browser/android/vr_shell/vr_shell.cc

@@ -857,6 +857,9 @@ void VrShell::ProcessContentGesture(std::unique_ptr<blink::WebInputEvent> event,
   if (content_id_ != content_id)
     return;
 
+  if (!android_ui_gesture_target_)
+    return;
+
   android_ui_gesture_target_->DispatchWebInputEvent(std::move(event));
 }