[DevTools] Handle zoom via InspectorFrontendHost.

TBR=pfeldman@chromium.org
BUG=343816

Review URL: https://codereview.chromium.org/166713002

git-svn-id: svn://svn.chromium.org/blink/trunk@167241 bbb929c8-8fbe-4397-9dbb-9b2b20218538