Unthrottled, the CPU can generate and queue an increasingly large number
of frames with the GPU lagging behind, especially with vsync enabled.

Assuming N swapchain images, this commit adds a wait on fence for the
Nth previous frame before submitting new work, that is the CPU is
always at most N frames ahead of the GPU.

Bug: angleproject:2908
Change-Id: Ieb2bf20168bfe9bc9d8e2219f682b01347c21dec
Reviewed-on: https://chromium-review.googlesource.com/c/1296953
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>