DevTools: ResourceTreeFrame._removeChildFrames is O(n^2).

BUG=None
R=vsevik@chromium.org, yurys@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22410005

git-svn-id: svn://svn.chromium.org/blink/trunk@155691 bbb929c8-8fbe-4397-9dbb-9b2b20218538