Bug 1063992 - Set the animation player hold time to zero when it is initially paused; r=dzbarsky

This patch fixes a regression from bug 1033114, m-c changeset 9db3e43c19c1.

That changeset changed the meaning of mHoldTime (despite the commit message
which erroneously refers to mStartTime) to make it an offset from the start time
rather than a timestamp. However, it failed to update the case when we have an
initially-paused player. In that case the offset should be zero but the existing
code set it to the same value as the start time (which is, itself, an offset
from the beginning of the timeline) and the above changeset failed to update
that.