Only creates outermost main frame's FrameSink on prerender cc warm up

Follow-up of crrev.com/c/5392245.

Based on the discussion on [1], this CL limits the signal of
WarmUpCompositor to widget only from outermost main frame.

[1] crrev.com/c/chromium/src/+/5595289/comment/064c0cc9_544298c9/

Bug: 41496019, 40240492
Change-Id: Ic3e3245ce5eb2c78c9c9c1c61faf72e0d0398c74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5611412


Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Lingqi Chi <lingqi@chromium.org>
Commit-Queue: Taiyo Mizuhashi <taiyo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312807}