Fix RemoteFrame detachment to signal its client.

We were seeing crashes in checkFrameCountConsistency() because during
RemoteFrame detachment the client was not getting signalled to modify
the frame tree, resulting in inconsistent state. This CL corrects that
problem.

BUG=430972

Review URL: https://codereview.chromium.org/712653002

git-svn-id: svn://svn.chromium.org/blink/trunk@184987 bbb929c8-8fbe-4397-9dbb-9b2b20218538