Bug 847275 patch 1:  Make assertions cause test failures in browser-chrome mochitest.  r=dao

I tested all three error messages manually by making different
SimpleTest.expectAssertions() calls in
browser/base/content/test/browser_aboutHome.js .
Bug 847275 patch 2:  Temporarily disable failures from checking of assertions in browser-chrome mochitests so that we can annotate the expected assertions before enabling.  r=dao
Bug 900783:  Make HasAnimationOfProperty return false in the refresh driver tick in which the transition completes.  r=nrc

I don't know of any observable bug that this fixes, but the code without
this fix seems incorrect; the "removed sentinel" concept generally
requires that callers enumerating transitions check that they're not
enumerating the sentinel.

This ensures that HasAnimationOfProperty switches from returning true to
false in the first refresh cycle after the end of the animation rather
than the second.

I originally wrote this in
https://bugzilla.mozilla.org/show_bug.cgi?id=876626#c13 but it turned
out not to be related to that bug.
Bug 906378:  Make the test for transitions not being supported in test_transitions_per_property.html only use about 50 values for each property to avoid the O(N^2) case blowing up.  r=dholbert

This currently causes a reduction in the number of background-image
values tested (by 54810, a majority of the assertions in the test file).
It doesn't change the number of values tested for any other property.