StandardQueryGL flushes with every pause/resume/end operation to make
sure the list of pending queries does not grow too large. This ended
up trying to flush the queries as soon as they were ended and unlikely
to have results ready.

Add a threshold of pending queries before flushing, hopefully delaying
the flush calls until the queries are ready.

This is a speculative fix for a GPU hang in query flushing.

Bug: chromium:1078754
Change-Id: I38300995ee6576dee8c8c26fa9ccad26e269337a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2401178Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>