[VizHitTesting] Update HitTestQueryFuzzer to create valid FrameSinkId

The fuzzer may produce invalid FrameSinkId for hit test regions which
incorrectly breaks a DCHECK in HitTestQuery.

Bug: 1001552
Change-Id: I0207dc7c1594f847e8fd3801c121ff04cd938e13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796114
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696034}