DevTools: NetworkRequest: convert connectionId to string

String is much more useful, it can be used as key for StringMap.

BUG=

Review URL: https://codereview.chromium.org/556363002

git-svn-id: svn://svn.chromium.org/blink/trunk@181763 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Detach all subframes before swapping a frame.

BUG=357747

Review URL: https://codereview.chromium.org/544443002

git-svn-id: svn://svn.chromium.org/blink/trunk@181764 bbb929c8-8fbe-4397-9dbb-9b2b20218538