Add runtime flag to enable/disable CSS variables (in addition to existing compile...

Add runtime flag to enable/disable CSS variables (in addition to existing compile-time flag).
https://bugs.webkit.org/show_bug.cgi?id=89542

Reviewed by Dimitri Glazkov.

Source/WebCore:

Added fast/css/variables/build-supports-variables.html to test runtime flag in test environment.
Updated all other css variables tests to enable the runtime flag from JS.

* css/CSSParser.cpp:
(WebCore::CSSParserContext::CSSParserContext):
(WebCore::operator==):
(WebCore::CSSParser::cssVariablesEnabled):
(WebCore):
(WebCore::CSSParser::detectDashToken):
(WebCore::CSSParser::lex):
* css/CSSParser.h:
(CSSParser):
* css/CSSParserMode.h:
(CSSParserContext):
* page/Settings.h:
(Settings):
(WebCore::Settings::setCSSVariablesEnabled):
(WebCore::Settings::cssVariablesEnabled):

Source/WebKit/chromium:

* features.gypi:
* public/WebSettings.h:
* src/WebSettingsImpl.cpp:
(WebKit::WebSettingsImpl::setExperimentalCSSVariablesEnabled):
(WebKit):
* src/WebSettingsImpl.h:
(WebSettingsImpl):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121229 268f45cc-cd09-0410-ab3c-d52691b4dbfc