[DevTools] Add support for 'Embedder Callback' event type in Timeline

This event is to be used for embedder callback events that occur when
Chromium is embedded, e.g. in the case of Android WebView.

BUG=344871
R=caseq@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/172213002

git-svn-id: svn://svn.chromium.org/blink/trunk@167601 bbb929c8-8fbe-4397-9dbb-9b2b20218538