Remove property-specific handling in animation-test-helpers.js

This patch makes animation-test-helpers.js not care about which
properties are being tested by always working with strings from computed
style directly. The previous behaviour relies heavily on the no longer
web-exposed getPropertyCSSValue which we want to delete.

Instead of having separate expectation types for different properties,
we simply match numeric substrings of the computed style value. The top
of the file has a comment explaining in more detail.

BUG=428595

Review URL: https://codereview.chromium.org/716963002

git-svn-id: svn://svn.chromium.org/blink/trunk@185179 bbb929c8-8fbe-4397-9dbb-9b2b20218538