[Resource Timing] Add Experimental Feature Flag

[Resource Timing] Add Experimental Feature Flag
https://bugs.webkit.org/show_bug.cgi?id=167147

Reviewed by Ryosuke Niwa.

Source/WebKit2:

* Shared/WebPreferencesDefinitions.h:
Move ResourceTiming to experimental section.
Disable it for El Capitan due to known issues.

Tools:

* DumpRenderTree/mac/DumpRenderTree.mm:
(enableExperimentalFeatures):
(resetWebPreferencesToConsistentValues):
* DumpRenderTree/win/DumpRenderTree.cpp:
(enableExperimentalFeatures):
(resetWebPreferencesToConsistentValues):
Put experimental flags in the experimental section.

Websites/webkit.org:

* experimental-features.html:
Test for Resource Timing.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@212945 268f45cc-cd09-0410-ab3c-d52691b4dbfc