Bug 1048838 - Properly invalidate the style rule in nsTransitionManager::StyleContextChanged.  r=dbaron

This patch fixes a regression from
https://hg.mozilla.org/mozilla-central/rev/31695984cfe2 (bug 1025709). That
patch replaced the EnsureStyleRuleFor method on ElementTransitions and
ElementAnimations with a common method in CommonElementAnimationData.

ElementTransitions::EnsureStyleRuleFor would create a new style rule if
there was no style rule (mStyleRule == nullptr) or if the refresh time was
old (mStyleRuleRefreshTime != aRefreshTime).

ElementAnimations::EnsureStyleRuleFor, however, would create a new style rule
only if mStyleRuleRefreshTime was null or old since a null style rule may
still be valid for animations (unlike transitions). If we bail as soon as we
a null style rule we would never update mNeedsRefreshes when the animation
finishes.

The unified version of EnsureStyleRuleFor in CommonElementAnimationData
adopted the behavior from ElementAnimations checking for a null or old
mStyleRuleRefreshTime.

However, nsTransitionManager::StyleContextChanged sets mStyleRule to nullptr
to indicate that we need to generate a new style rule. This means that we
will fail to create a style rule for the transition in some cases.

This patch addresses this by making nsTransitionManager::StyleContextChanged
set mStyleRuleRefreshTime to a null timestamp. Setting mStyleRule to nullptr
is no longer necessary since EnsureStyleRuleFor will do this when necessary
and nsTransitionManager::mStyleRule is only used after calling
EnsureStyleRuleFor.
Bug 1048838 - Add mochitest for transitions and restyles.  No review.

The third test fails without the patch, and passes with the patch.