[vsync] Add ExternalBeginFrameSourceWinUsesRunOrPostTask to testing config.

Bug: 40944462
Change-Id: Ib2c580710c617b47e06f92ef753efcbdddb03651
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5502696


Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312564}