Refactor WebGraphicsContext3DVideoFramePool to return SyncToken

WebGraphicsContext3DVideoFramePool::CopyRGBATextureToVideoFrame()
previously returns a boolean. This CL refactors this function to
return an optional SyncToken, in accordance with a similar change
in media::CopyRGBATextureToVideoFrame (https://crrev.com/c/6378003).

Bug: 40286368
Change-Id: I51124d97b462c6073b4820299618263c13de77e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6382805
Commit-Queue: Mingjing Zhang <mjzhang@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1437033}