Web Animations CSS: Skip checking for new or updated animations when in an animation style change

When recalculating the style for an element only for the purposes of animation,
no animations can start, be cancelled, or have their play state changed. Use
the hook added in r164013[1] to skip this logic, but verify that it's correct
when running in debug.

This exposed a subtle problem with the animation-style-change mechanism. When
an element lost it's renderer (eg. due to an ancestor being marked
display:none) and then gained one again it was possible that the
animation-style-change flag was never cleared. Clearing of this flag was added
to detach/attach and is covered by the existing test in [2].

[1] https://src.chromium.org/viewvc/blink?view=rev&revision=164013
[2] animations/display-none-cancels-nested-animations.html

Review URL: https://codereview.chromium.org/100023008

git-svn-id: svn://svn.chromium.org/blink/trunk@164061 bbb929c8-8fbe-4397-9dbb-9b2b20218538