Web Animations CSS: Adding ASSERT for the timing function in Timing object.
Web Animations CSS: Adding ASSERT for the timing function in Timing object.

Alternative version would be to populate the timing function in the contructor
with something like this;

BUG=

Review URL: https://codereview.chromium.org/70233007

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@162178 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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