Implement a memory limit for subframe process reuse.

Currently, new out-of-process subframes will aggressively attempt to
reuse any available process with a compatible ProcessLock. In certain
situations, such as several tabs with heavy OOPIFs from the same site,
this unrestricted reuse may lead to an overly large process that
eventually runs out of memory.

This CL adds a feature, SubframeProcessReuseThresholds, which applies
a memory threshold for subframe process reuse. That is, a process will
only be allowed to be reused for a new OOPIF if it hasn't reached a
predefined, experiment-controlled memory threshold (with the default
value of 2GB). Only the REUSE_PENDING_OR_COMMITTED_SITE_SUBFRAME reuse
policy will be affected by this change. In the future, we may consider
adding other restrictions on subframe process reuse, e.g. with
heuristics such as the process's responsiveness.

Bug: 40600119, 40259123
Change-Id: If43a3e2d02023cd91f8089b73bf03282b63b8f77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5843350
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1354115}