Remove superfluous TimingFunction::isXXXTimingFunction() helpers

In most cases it's better to use TimingFunction::type() in a switch statement.

R=dstockwell

Review URL: https://chromiumcodereview.appspot.com/23522007

git-svn-id: svn://svn.chromium.org/blink/trunk@157053 bbb929c8-8fbe-4397-9dbb-9b2b20218538