Rewrite VsyncRefreshDriverTimer::GetTimerRate to always use the cached
value of the vsync rate in VsyncChild to avoid processing events on
the main thread.
Since VsyncChild::GetTimerRate is called in VsyncRefreshDriverTimer's
constructor, that cached value is bound to be set soon. This should
make the period of time we need to guess in
VsyncRefreshDriverTimer::GetTimerRate very short.
MozReview-Commit-ID: 1bnHNXAP8jY
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extra : rebase_source : 5a731962d417c4b3352970b2adb92b5d31de021c
5ae81e4455