[WebGL] Move source buffer to ReadbackFramebuffer

There are two call-sites that prepare framebuffer outside, one can
handle both back and front, other can handle only back.

Move this logic inside the function, so we don't need to dup it.

Change-Id: I9ec00f9f08b8f47db69eb9cace1af2894fbd00a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6792203
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1493063}