[CSS Grid Layout] Enable runtime feature if feature flag is enabled

[CSS Grid Layout] Enable runtime feature if feature flag is enabled
https://bugs.webkit.org/show_bug.cgi?id=130013

Reviewed by Simon Fraser.

Source/WebKit/mac:

Enable CSS Grid Layout runtime feature when ENABLE_CSS_GRID_LAYOUT flag is true.

* WebView/WebPreferences.mm:
(+[WebPreferences initialize]):

Source/WebKit2:

Enable CSS Grid Layout runtime feature when ENABLE_CSS_GRID_LAYOUT flag is true.

* Shared/WebPreferencesStore.h:

LayoutTests:

Modify layout test as feature is not disabled by default in WK2.
The test now checks that the runtime feature can be disabled.

* fast/css-grid-layout/grid-disabled-expected.txt: Renamed from LayoutTests/fast/css-grid-layout/grid-disabled-by-default-expected.txt.
* fast/css-grid-layout/grid-disabled.html: Renamed from LayoutTests/fast/css-grid-layout/grid-disabled-by-default.html.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166614 268f45cc-cd09-0410-ab3c-d52691b4dbfc