Add 'getBacktrace' to protocol

This is a successor for https://chromiumcodereview.appspot.com/19064004/

Review URL: https://chromiumcodereview.appspot.com/19543015

git-svn-id: svn://svn.chromium.org/blink/trunk@154658 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert 'anti-breakpoints' in UI

Review URL: https://chromiumcodereview.appspot.com/19700004

git-svn-id: svn://svn.chromium.org/blink/trunk@154659 bbb929c8-8fbe-4397-9dbb-9b2b20218538