Bug 1104424 - Rework pausing in AnimationPlayer to remove mIsPaused; r=dholbert

This patch reworks AnimationPlayer to represent the paused state by a null start
time. This brings it into line with recent changes in the Web Animations spec
and removes the need for the mIsPaused member variable.

The idea is that in order for a player to play, it needs a start time and
an active timeline. The processing is roughly:
* If it is seeked to a particular time (through setting the currentTime or
  calling pause() or finish()) but has no start time, it is paused.
* Otherwise, if it has no active timeline or no start time, it is idle.

By removing the mIsPaused flag the number of possible permutations of states is
reduced so the model is easier to reason about (see:
http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0026.html).

This patch replaces the mIsPaused flag with checks for mStartTime.IsNull()
according to the rules outlined above.
Bug 1104427 part 1 - Rename GetStartTime to GetStartTimeAsDouble; r=bholley

In forthcoming patches we will encapsulate AnimationPlayer::mStartTime so we can
ensure that related state is updated appropriately. We would like to expose
mStartTime via GetStartTime() but currently a method of that name returns the
start time as a double.

This patch applies the pattern used for currentTime to startTime; specifically,
GetCurrentTime() returns the TimeDuration (since that's what C++ callers should
use) while GetCurrentTimeAsDouble() returns a double.

At the same time, this patch also removes the [Pure] extended attribute from
startTime in the WebIDL definition since subsequent patches either in this bug
or in bug 927349 will mean that startTime can be updated out-of-band.
Specifically, we will implement deferred playback of animation such that the
startTime remains null until we've finished rendering the first frame of the
animation.
Bug 1104427 part 2 - Encapsulate AnimationPlayer::mStartTime; r=dholbert

Now that there is a public accessor for mStartTime, we can make it a protected
member of AnimationPlayer. The only time mStartTime is ever set is when playing
the animation so we can replace external modifications to mStartTime with calls
to Play(). This simplifies implementing deferred starting of animations
in bug 927349 by isolating the deferred playback logic to AnimationPlayer.

Note that even when we call PauseFromStyle immediately afterwards we still need
to call PlayFromStyle (or Play) first in order to resolve the time at which the
player should be paused. A newly created player doesn't have a current time so
if we were simply to call pause it wouldn't pause at the start of the animation
as we might expect. The call to Play(FromStyle) will cause the current time to
become zero and then we pause at that time.
Bug 1104427 part 3 - Make resolving the start time a separate method in AnimationPlayer; r=dholbert

In preparation for deferring the start of animations in bug 927349, this patch
makes the code that resolves the start time a separate method. In this patch we
call the method immediately from AnimationPlayer::DoPlay. However, in the future
this will be called at a later point when the first frame of the animation has
been rendered.