Check for null return from layerTreeView() in DevToolsEmulator.
Check for null return from layerTreeView() in DevToolsEmulator.

Fixes Chromium unit test failures.

BUG=515746

Review URL: https://codereview.chromium.org/1264703005

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199983 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk