Quota: Remove StorageMonitor class.

While adding class comments to Quota classes, I noticed that there are
no calls to StorageMonitor::AddObserver() outside of tests. Without any
observers, this class serves no other purpose.

Change-Id: I9066cbddad78b10de2eb1082cc3bb381e334cd9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1679156
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675728}
13 files changed