Workaround for NPE in UrlBar#onTouchEvent()

BUG=757161

Change-Id: Icc9e4a60731cd3f18da1f966fd54ae7733f0366f
Reviewed-on: https://chromium-review.googlesource.com/781103


Reviewed-by: Maria Khomenko <mariakhomenko@chromium.org>
Commit-Queue: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518266}