[DevTools] Fix regression introduced in r174361.

After switching from insets to bounds, we are dependent on the right
window size when docking/undocking.
So, we should not update page bounds until we got final window size.

BUG=373669

Review URL: https://codereview.chromium.org/287053005

git-svn-id: svn://svn.chromium.org/blink/trunk@174588 bbb929c8-8fbe-4397-9dbb-9b2b20218538