Remove remaining SetPlatformSource calls from mediacapturefromelement/

Part of an effort to remove SetPlatformSource from MediaStreamSource,
instead using a new constructor, to better align their lifecycles.

#removeplatformsetters

Bug: 1302689
Change-Id: I00e783e1cc37a60c8a7de624fccbabc9b2b5a982
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3557230
Commit-Queue: Tony Herre <toprice@chromium.org>
Auto-Submit: Tony Herre <toprice@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#986751}