[Web Animations] Tests using the new animation engine may crash under WebCore::FrameV...

[Web Animations] Tests using the new animation engine may crash under WebCore::FrameView::didDestroyRenderTree when using internals methods
https://bugs.webkit.org/show_bug.cgi?id=185612
<rdar://problem/39579344>

Reviewed by Dean Jackson.

Source/WebCore:

Add a new internals.pseudoElement() method to obtain a pseudo element matching a given pseudo-id. This is necessary to be able to move off
internals.pauseTransitionAtTimeOnPseudoElement() and internals.pauseAnimationAtTimeOnPseudoElement() for Web Animations testing.

* testing/Internals.cpp:
(WebCore::Internals::pseudoElement):
* testing/Internals.h:
* testing/Internals.idl:

LayoutTests:

Some tests that were opting into the new animation engine were using internals methods (pauseAnimationAtTimeOnElement, pauseTransitionAtTimeOnElement, etc.)
that enforce the creation of animations in the old animation engine. Meanwhile, the code that toggles the animation engine used based on HTML comments is run
prior to teardown of the previous test and so a test running with the new engine would run with the legacy engine during teardown. These two factors would
cause `ASSERT(!frame().animation().hasAnimations())` to fail under FrameView::didDestroyRenderTree().

We update tests that use these internals method to use the Web Animations API instead and opt into the new animation engine if they didn't already do that.

* animations/animation-hit-test-transform.html:
* animations/keyframes-dynamic-expected.txt:
* animations/keyframes-dynamic.html:
* animations/missing-from-to-expected.txt:
* animations/missing-from-to-transforms-expected.txt:
* animations/missing-from-to-transforms.html:
* animations/missing-from-to.html:
* fast/css-generated-content/pseudo-animation.html:
* transitions/transition-hit-test-transform.html:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@231766 268f45cc-cd09-0410-ab3c-d52691b4dbfc