[DevTools] Speculative fix for crash in InspectorFrontendHost::zoomFactor.

BUG=400483

Review URL: https://codereview.chromium.org/513153002

git-svn-id: svn://svn.chromium.org/blink/trunk@181045 bbb929c8-8fbe-4397-9dbb-9b2b20218538