DevTools: mark view as shown in _processWillShow, not in _processWasShown

This assures that whatever views are shown as children of the view being
shown from within its wasShown method will see that the parent is already
shown and will thus have both their _processWillShow() and _processWasShown()
executed.

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/20630004

git-svn-id: svn://svn.chromium.org/blink/trunk@154994 bbb929c8-8fbe-4397-9dbb-9b2b20218538