DevTools: factor out InspectorFrontendHost/API as a separate entity:
- removes old event sink
- moves interface definition out of externs
- moves Platform, UserMetrics into host
- fixes dependencies

Review URL: https://codereview.chromium.org/381643002

git-svn-id: svn://svn.chromium.org/blink/trunk@177756 bbb929c8-8fbe-4397-9dbb-9b2b20218538