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This exemption allowed the fullscreen element stack to get into a state
not otherwise possible, namely with elements in any order as opposed to
in tree order. It also seemed possible to enter fullscreen in a sibling
iframe, leading to a (cross-document) set of fullscreen elements forming
a tree instead of a simple chain, a case without any test coverage.

Removing the exemption means that a webkitfullscreenerror event will be
fired if one is already in fullscreen and requests fullscreen for an
element other than a descendant of the current fullscreen element.

The use counter was intended to tell whether any content depends on this
or not, but data will not be available for a long time yet. In order to
be affected one needs to, in fullscreen, have a currently visible button
request fullscreen for an element currently not visible, which seems
far-fetched, so site compat is probably not a problem.

The use counter for webkitRequestFullScreen() provides an upper bound:
http://www.chromestatus.com/metrics/feature/timeline/popularity/177

BUG=383813

Review URL: https://codereview.chromium.org/458223003

git-svn-id: svn://svn.chromium.org/blink/trunk@180243 bbb929c8-8fbe-4397-9dbb-9b2b20218538