Use CrossThreadBindOnce in MediaStreamVideoCapturerSourceTest

... when posting tasks. A task posted runs once, so
CrossThreadBindRepeating is not strictly correct.

This is now possible, after [1].

[1] crrev.com/c/1750043

BUG=964922
R=jbroman@chromium.org

Change-Id: Ic1937f017ec6cd5a532b99a1f270f2ce2a99b8dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752078
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686674}
1 file changed