Disable opt-in origin isolation when under the memory threshold.

This CL adds logic to not honor opt-in origin isolation (typically
specified via the Origin-Isolation header) if the environment has a
site isolation memory threshold in effect, and the device is below
that threshold.  Memory thresholds are used on Android, where the
default threshold is currently 1900MB (overridable via field trials),
and where full site isolation is disabled by default.  They aren't
used on desktop, so this change does not apply to desktop platforms.

An exception is made to still honor opt-in origin isolation if full
site isolation is in effect, such as if the user manually turned it on
via chrome://flags or if this was done via enterprise policy.
Similarly, if the user manually turns off site isolation via
--disable-site-isolation-trials (or the equivalent chrome://flags
entry), that also disables opt-in origin isolation, including on
desktop platforms.

Bug: 1145860
Change-Id: Ibc5b2f6100ea9f6e2e46e25cefb52ba51ac697f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520086


Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824763}