Remove redundant forward declaration and header inclusions.

Remove forward declaration from HTMLStyleElement.h - StyleSheet
Remove header inclusions from HTMLStyleElement.cpp - ContextFeatures.h, StyleEngine.h, StyleSheetContents.h

NOTRY=true

Review URL: https://codereview.chromium.org/405563002

git-svn-id: svn://svn.chromium.org/blink/trunk@178552 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Predeclare globals required by the test framework to avoid walking the prototype chain.

Otherwise http/tests/security/xss-DENIED-method-with-iframe-proto.html double-faults while trying to read those global variables that aren't declared.

BUG=

Review URL: https://codereview.chromium.org/405893003

git-svn-id: svn://svn.chromium.org/blink/trunk@178553 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add the HTMLIFrameElement.allowFullscreen reflected attribute

http://whatwg.org/html#dom-iframe-allowfullscreen

BUG=383813

Review URL: https://codereview.chromium.org/398843002

git-svn-id: svn://svn.chromium.org/blink/trunk@178555 bbb929c8-8fbe-4397-9dbb-9b2b20218538