Remove the webkitallowfullscreen content attribute

Intent to Deprecate and Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/LupOLlogL-g/H8GsBEDGQeMJ

Tests were updated to use allowfullscreen instead of
webkitallowfullscreen, with a few exceptions:

fullscreen/full-screen-enabled-prefixed.html would have become redundant
with fullscreen/full-screen-enabled.html and was removed.

fullscreen/full-screen-iframe-with-mixed-allow-webkitallow-attribute.html
became a test for nested iframes with allowfullscreen and was renamed.

Finally, doctypes were added and whitespace cleaned up in touched test.

fullscreen/resources/full-screen-iframe-allowed-nested2.html depended on
being in quirks mode, since the size of the body element is smaller in
standards mode. Put the click event handler on window instead.

BUG=390008

Review URL: https://codereview.chromium.org/336313015

git-svn-id: svn://svn.chromium.org/blink/trunk@177332 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move core/WebCorePrefixMac.h to build/mac/Prefix.h to make it a top level build prefix.

Review URL: https://codereview.chromium.org/360353002

git-svn-id: svn://svn.chromium.org/blink/trunk@177333 bbb929c8-8fbe-4397-9dbb-9b2b20218538