This patch (finally) puts pausing animations in the pending player map so that
they are resolved asynchronously.
Since this changes the pausing behavior this patch updates a number of tests so
that they continue to pass.
AnimationPlayer.ready will currently always be resolved but by updating these
tests to wait on it anyway they should continue to work once we introduce the
delayed animation start behavior.
Since bug 1104433, step_func now passes on the return value of its callback.
That means we can use it directly as a Promise callback function in Promise
chains where the return value of the function is another Promise.
This patch updates existing tests to eliminate the awkward handling we had
around wrapping some parts of Promise callbacks in step() but leaving the return
statement outside it.
This patch moves commonly used addDiv and waitForFrame test methods to
a separate testcommon.js support file.
It also takes advantage of the updates to testharness.js from bug 1104433 to
rework addDiv such that it automatically removes the created div at the end of
the test.
