Add feature parameter to throttle frame rate when the loading starts

The CL adds an extra feature parameter to throttle the frame rate by
default. The frame rate will always be unthrottled when the document
parsing ends with the feature parameter on. The parameter is introduced
for experiments to measure the maximum potential impact of frame rate on
page loading.

Bug: 397832388

Change-Id: I9dc7082b983627a66c1e0be857e8db98967453ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6332452
Commit-Queue: Jiacheng Guo <gjc@google.com>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430075}