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}
1 file changed