A page should exit fullscreen mode if it opens a new popup

If a fullscreen page opens a popup, the popup would be hidden and
therefore invisible to the user. To avoid this, exit fullscreen mode
before opening a new window.

Imported from http://trac.webkit.org/changeset/157534

R=abarth@chromium.org
TEST=fullscreen/full-screen-exit-when-popup.html
BUG=304547

Review URL: https://codereview.chromium.org/27599002

git-svn-id: svn://svn.chromium.org/blink/trunk@159834 bbb929c8-8fbe-4397-9dbb-9b2b20218538