DevTools: introduce Target.exposeDevToolsProtocol() method

Method injects a binding object to the target's main frame that
allows communication with browser target.

R=pfeldman, dgozman

Change-Id: Ic36a058c2cba41fccb9419318676a4fdc04a8454
Reviewed-on: https://chromium-review.googlesource.com/1066289
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567104}
7 files changed