Bug 1104427 part 1 - Rename GetStartTime to GetStartTimeAsDouble; r=bholley

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.