Web Animations CSS: Support animation of {text,box,-webkit-box}-shadow and fix blur clamping

This patch adds support for animating the shadow properties in the
Web Animations implementation. We fix the interpolation of blur to
clamp to zero, but leave the existing incorrect behaviour in which
shadow lists with mismatched shadows will interpolate anyway, even
though the spec suggests (it is poorly written!) that we shouldn't
be doing any interpolation here.

The code for interpolating ShadowLists in CSSPropertyAnimation has
been moved to ShadowList and ShadowData, so we can re-use it, with
minor stylistic changes.

Note that some of the test expectation failures are due to integer
rounding, which https://codereview.chromium.org/23241010/ probably
will resolve.

BUG=257591

Review URL: https://codereview.chromium.org/38823002

git-svn-id: svn://svn.chromium.org/blink/trunk@160421 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove LayoutTest animations/animation-direction-reverse-fill-mode-pause.html

This test is incorrect and flaky, and it's not clear what it's trying to test.

This test checks the animated value after the animation has ended. Since the
animation does not use fill, the animated value should be the underlying value,
but the test checks for the 'to' value.

This test is also inherently flaky, as it uses the runAnimationTest() helper
without the internals pause API. While it is not currently failing on the bots,
it fails repeatably when run locally in isolation.

Finally, it's not clear what the test is trying to test. It pauses the
animation after it has completed, but it's not clear why. The other aspects of
the test are well covered in other tests.

R=dstockwell

Review URL: https://codereview.chromium.org/39373002

git-svn-id: svn://svn.chromium.org/blink/trunk@160422 bbb929c8-8fbe-4397-9dbb-9b2b20218538