[css exclusions] Enable CSS Exclusions by default when CSS_EXCLUSIONS is set
https://bugs.webkit.org/show_bug.cgi?id=115919

Reviewed by Dean Jackson.

Source/WebCore:

Change the RuntimeEnabledFeatures::isCSSExclusionsEnabled boolean to default
to true. CSS Exclusions can still be enabled / disabled in platforms by
setting the CSS_EXCLUSIONS compile flag in one of the FeatureDefine files.

Test: fast/exclusions/css-exclusions-enabled.html

* bindings/generic/RuntimeEnabledFeatures.cpp:
(WebCore):

LayoutTests:

Adding a test to make sure CSS Exclusions can be enabled, and are enabled by
default.

* fast/exclusions/css-exclusions-enabled-expected.txt: Added.
* fast/exclusions/css-exclusions-enabled.html: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150025 268f45cc-cd09-0410-ab3c-d52691b4dbfc