[DevTools] Register (crafted) child frames.

Some iFrames are crafted (created with document.write).
Those do not cause "didCommitLoad".
Consequently, frames inside crafted frames can't be properlt
bound to their parents.

To fix this we notify inspector about new child frames.

(In distinct patch we can add safeguard to prevent "main frame navigated"
in case frame wasn't properly bound to its parent)

BUG=259036

Review URL: https://chromiumcodereview.appspot.com/18698006

git-svn-id: svn://svn.chromium.org/blink/trunk@154739 bbb929c8-8fbe-4397-9dbb-9b2b20218538