[DevTools] Remove unused embedder messages.

BUG=none

Review URL: https://codereview.chromium.org/749473002

git-svn-id: svn://svn.chromium.org/blink/trunk@185868 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[DevTools] Cleanup legacy host methods.

BUG=none

Review URL: https://codereview.chromium.org/744873002

git-svn-id: svn://svn.chromium.org/blink/trunk@185869 bbb929c8-8fbe-4397-9dbb-9b2b20218538