Browsing Data: Add README.md to explain what "canned" data is

I can never remember what role the CannedBrowsingDataXYZHelper classes
serve when someone asks, so documenting it for posterity.

TBR=dullweber@chromium.org,mxcai@chromium.org
NOTRY=true

Change-Id: Id5853e0baccc2dd1b3649a6398cb3e86e1c1f5a1
Reviewed-on: https://chromium-review.googlesource.com/c/1431234
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625344}

chrome/browser/browsing_data/README.md

+# CookiesTreeModel
+
+A CookiesTreeModel is instantiated in multiple places in Chrome:
+
+* "All cookies and site data" (chrome://settings/siteData)
+* "All sites" (chrome://settings/content/all)
+* "Cookies in use" display off the origin chip in the infobar
+
+## BrowsingDataXYZHelper
+
+Instances of this type are used to fully populate a CookiesTreeModel
+with full details (e.g. origin/size/modified) for different storage
+types, e.g. to report storage used by all origins.
+
+When StartFetching is called, a call is made into the relevant storage
+context to enumerate usage info - usually, a set of tuples of (origin,
+size, last modified). The CookiesTreeModel assembles this into the
+tree of nodes used to populate UI.
+
+Some UI also uses this to delete origin data, which again calls into
+the storage context.
+
+## CannedBrowsingDataXYZHelper
+
+Subclass of the above. These are created to sparsely populate a
+CookiesTreeModel on demand by LocalSharedObjectContainer, with only
+some details (e.g. full details for cookies, but only the usage of
+other storage typess).
+
+* TabSpecificContentSettings is notified on storage access/blocked.
+* It calls into the "canned" helper instance for the storage type.
+* The "canned" instance records necessary "pending" info about the access.
+* On demand, the "pending" info is used to populate a CookiesTreeModel.
+
+This "pending" info only needs to record the origin for most storage
+types.