[DevTools] Strenghten up backward compatibility for remote frontends.

This change moves installing convenient API atop of
InspectorFrontendHost.sendMessageToEmbedder to InspectorFrontendClientImpl
to maintain backward-compatibility architecture.

If we do the same in javascript, any compatibility stuff may be
overridden in javascript later.

BUG=none

Review URL: https://codereview.chromium.org/136463005

git-svn-id: svn://svn.chromium.org/blink/trunk@165850 bbb929c8-8fbe-4397-9dbb-9b2b20218538