[DevTools] Merge InspectorFrontendClientImpl into WebDevToolsFrontendImpl and remove unused code.

BUG=none

Review URL: https://codereview.chromium.org/481913002

git-svn-id: svn://svn.chromium.org/blink/trunk@180464 bbb929c8-8fbe-4397-9dbb-9b2b20218538