DevTools: Notify InspectorFrontendHost about the frontend having loaded.

This requires backend support introduced in https://codereview.chromium.org/624193003/.
Until that has landed, this change is a no-op.

R=pfeldman
BUG=420453

Review URL: https://codereview.chromium.org/636493003

git-svn-id: svn://svn.chromium.org/blink/trunk@183324 bbb929c8-8fbe-4397-9dbb-9b2b20218538