Quota: Add a basic README.

Bug: 1102433
Change-Id: I65e8d1504b5ffdcd34706bba746db132692f2d9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2589008
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836808}

storage/browser/quota/README

+# Overview
+
+The quota system's primary role is to set and enforce limits on disk usage at
+both the browser level, and at the origin level (see ./quota_settings.cc for
+these limit values). The quota system manages disk usage only for certain web
+platform storage APIs.
+
+In order for a storage backend to integrate with the quota system, it must
+implement the QuotaClient interface.
+
+Most work on the quota system is currently done on the browser process' IO
+thread.  There are plans for quota to be moved to [the Storage
+Service](https://docs.google.com/document/d/1v83XKVxnasgf2uNfb_Uc-rfhDa3-ynNP23yU2DWqshI/),
+which will run on its own process on desktop platforms.
+
+# Key Components
+## Interface
+The quota system's interface is comprised of the following classes:
+
+### QuotaManager
+The "heart" of the quota system. This class lives on the browser
+process' IO thread, but is primarily accessed through QuotaManagerProxy, which
+handles thread hops. In the future, QuotaManagerProxy will turn into
+mojom::QuotaManager, and the QuotaManager class will become QuotaManagerImpl.
+
+### QuotaClient
+This interface must be implemented by any storage backend that wants to
+integrate with the quota system. This is probably the most used interface from
+outside of the quota system.
+
+### PaddingKey
+Helpers for computing quota usage for opaque resources. Features that store
+opaque resources (AppCache, Cache Storage) should use these helpers to avoid
+leaking cross-origin information via the quota usage they report.
+
+### SpecialStoragePolicy
+Hook that allows browser features (currently Extensions and Chrome Apps) to
+change an origin's quota.
+
+## Implementation
+The quota system's implementation is made up of the following components:
+
+### UsageTracker, ClientUsageTracker
+QuotaManager helpers that distribute tasks (e.g. measure an origin's quota
+usage) across QuotaClient instances, and cache results as needed.
+
+### QuotaDatabase
+Stores persistent information in a per-profile SQLite database. Currently stores
+a few bits of implementation details, and will likely be expanded to cover
+Storage Buckets. The currently stored information is a usage count,
+last-modified-time, and last-accessed-time for each origin (used to implement
+LRU eviction on storage pressure, and Clear Site Data with a time filter), and
+quota granted via the deprecated API
+webkitStorageInfo.requestQuota(PERSISTENT,...).
+
+### QuotaTemporaryStorageEvictor
+Handles eviction and records stats about eviction rounds.
+
+### QuotaTask
+Implementation detail of QuotaManager.
+
+# Glossary
+
+## Storage Pressure
+A device is said to be under storage pressure when it is close to capacity.
+Storage pressure is used to signal a couple of behaviors in the quota system:
+ - Eviction
+ - The QuotaChange event
+ - Triggering storage pressure UI (implementation specific)
+
+## Eviction
+This is the process by which the quota system cleans up app's data as disk usage
+gets close to the disk's capacity.
+
+# Resources
+ - [Chrome Web Storage and Quota Concepts](https://docs.google.com/document/d/19QemRTdIxYaJ4gkHYf2WWBNPbpuZQDNMpUVf8dQxj4U/)
+   - In-depth description of the quota system that also explains related
+     concepts and legacy APIs that left a mark on quota.