Move throttling-related classes to separate folder

This change moves ArcInstanceThrottle and ArcThrottleObserver
to chrome/browser/chromeos/arc/instance_throttle
Bug:b:133417350
Test: Build and flash to eve. Monitor /var/log/chrome/chrome to see throttling de/activate according to the different conditions.

Change-Id: Ied98988b65bc6a357601c90935e513fe276b6e12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762780
Commit-Queue: Willie Koomson <wvk@google.com>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689670}