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
We will use the new type for the generated IPDL message handler
prototype to make sure correct error handling method is called.
MozReview-Commit-ID: AzVbApxFGZ0
