Prerender: Perform pre-paint before prerender activation

For prerendering pages, they perform the style and layout
update phase before activation, which saved a lot of time.
This CL attempts to perform the prepaint phase on non-empty
pages before activation, behind a feature flag.

Bug: 336963892
Change-Id: I24cbf093f89289dd15f613f6ee2964988f068400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5436015
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Taiyo Mizuhashi <taiyo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1300448}