Set Main Thread's Runner on CopyOutputRequest if not already set

This is for SingleThreaded mode only (thus, ui/compositor only).

Bug: b/324199451
Change-Id: I6116a5829323c843622409757a26d4065911b8dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5287895


Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1270647}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5404293
Cr-Commit-Position: refs/heads/main@{#1282702}