Notify the embedder of v8::Context destruction before calling DetachGlobal()

This should reduce the chances of Blink or Chromium attempting to call
v8::Context::Global() after the global has been detached. In the long run,
the hope is to make calling Global() at that point an error, rather than
the current behavior of returning the raw global object.

It's also likely this will make the extensions code less weird, in that
it will be operating on a context that's in a reasonable state.

BUG=324812

Review URL: https://codereview.chromium.org/99983003

git-svn-id: svn://svn.chromium.org/blink/trunk@163112 bbb929c8-8fbe-4397-9dbb-9b2b20218538