Unprefix Page Visibility API.

Keep the prefixed API around, but add a UseCounter and print a deprecation
warning to the console. Behavior is unchanged except that code for a
non-standard "preview" state is removed; we never set that state anyway.
Update all the tests to use the unprefixed version and make a copy of one to
test the prefixed one.

For the visibilitychange event, both events are emitted. As this is just a
state-change notification, the failure modes of a site accidentally reacting to
both should not be bad.

Intent to Ship thread:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/TwCghLHBYwA

BUG=230334
TEST=fast/events/page-visibility-*

Review URL: https://codereview.chromium.org/59103013

git-svn-id: svn://svn.chromium.org/blink/trunk@161690 bbb929c8-8fbe-4397-9dbb-9b2b20218538