Should remove RuntimeEnabledFeatures::multipleShadowSubtreesEnabled

Should remove RuntimeEnabledFeatures::multipleShadowSubtreesEnabled
https://bugs.webkit.org/show_bug.cgi?id=79307

Reviewed by Dimitri Glazkov.

Source/WebCore:

Since RuntimeEnabledFeatures::multipleShadowSubtreesEnabled was introduced for
temporal unstability of multiple shadow roots. We should remove it now, and
enable it by default.

Test: fast/dom/shadow/shadow-root-js-api.html should cover this change.
The other shadow related tests should also covers this change.

* WebCore.exp.in:
* bindings/generic/RuntimeEnabledFeatures.cpp:
(WebCore):
* bindings/generic/RuntimeEnabledFeatures.h:
(RuntimeEnabledFeatures):
* dom/ShadowRoot.cpp:
(WebCore::ShadowRoot::create):
* dom/ShadowTree.cpp:
(WebCore::ShadowTree::addShadowRoot):
* testing/Internals.cpp:
* testing/Internals.h:
(Internals):
* testing/Internals.idl:

LayoutTests:

Removed multipleShadowSubtresEnabled from tests.

* fast/dom/shadow/get-element-by-id-in-shadow-root.html:
* fast/dom/shadow/multiple-shadowroot-adopt.html:
* fast/dom/shadow/multiple-shadowroot-rendering.html:
* fast/dom/shadow/multiple-shadowroot.html:
* fast/dom/shadow/shadow-element-rendering-multiple.html:
* fast/dom/shadow/shadow-element-rendering-single.html:
* fast/dom/shadow/shadow-root-js-api-expected.txt:
* fast/dom/shadow/shadow-root-js-api.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@111155 268f45cc-cd09-0410-ab3c-d52691b4dbfc