Web Animations CSS: Support animation of -webkit-mask-position

The current implementation does not animate mis-matched
-webkit-mask-position value counts correctly and is expected to fail
the newly added test.
The test passes under the Web Animations implementation.

BUG=257591

Review URL: https://codereview.chromium.org/26355006

git-svn-id: svn://svn.chromium.org/blink/trunk@159620 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Fix AnimatableImage to return unmodified keyframe values where possible when interpolating

Currently, if the interpolation fraction is outside the range [0, 1],
AnimatableImage clips the fraction to this range before performing the
cross-fade. This means that for interpolation fractions <=0 and >=1 the result
is a cross-fade with a fraction of 0 and 1 respectively. Instead we should
simply return the unmodified keyframe value for the relevant interpolation
fraction in these cases.

Also add tests for the following properties ...
- border-image-source
- list-style-image
- webkit-mask-box-image-source
- webkit-mask-image

Note that image interpolation that involves a gradient, while handled correctly
at the CSS level (in the case of the Web Animations implementation) is not
rendered by either the legacy or Web Animations implementations. See
http://crbug.com/306316.

BUG=257591
R=dstockwell

Review URL: https://codereview.chromium.org/26938004

git-svn-id: svn://svn.chromium.org/blink/trunk@159621 bbb929c8-8fbe-4397-9dbb-9b2b20218538