Merge 133429 - Shadow DOM should be able to be disabled per context.

Merge 133429 - Shadow DOM should be able to be disabled per context.
https://bugs.webkit.org/show_bug.cgi?id=101173

Reviewed by Dimitri Glazkov.

Source/WebCore:

This change pulls back relevant bits from r131549. Note that if
the the port enables runtime Shadow DOM flag, this can cause slow
down on some Chromium page cycler test cases (which r131549
attempted to fix).

This change is temporal and the flags should be switched back from
ContextFeatures to RuntimeEnabledFeatures once it gains sufficent
stability.

* dom/ContextFeatures.cpp:
(WebCore::ContextFeatures::shadowDOMEnabled):
(WebCore):
* dom/ContextFeatures.h:
* dom/Position.cpp:
(WebCore::Position::Position):
(WebCore::Position::findParent):
* dom/TreeScope.cpp:
(WebCore::TreeScope::getSelection):
* html/HTMLTagNames.in:
* html/shadow/HTMLContentElement.cpp:
(WebCore::HTMLContentElement::contentTagName):
* page/DOMWindow.idl:

Source/WebKit/chromium:

* src/ContextFeaturesClientImpl.cpp:
(WebKit::ContextFeaturesClientImpl::askIfIsEnabled):


TBR=morrita@google.com
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=134366