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This CL:
  1. changes the installation code to index all static rulesets provided
  by an extension.
  2. Adds unittests to ensure 1. above works as intended.

Remaining work:
  1. Currently rule limits for static rulesets are only evaluated per
  ruleset. Implement them for the set of enabled rulesets once we
  start supporting the 'enabled' property as part of rulesets specified
  in manifest.
  2. We have limits on the no. of install warnings per ruleset. Also,
  add a limit on the total number of warnings across all rulesets.
  3. Load multiple static rulesets on extension load.
  4. Provide ability to dynamically toggle the set of enabled static
  rulesets.

BUG=754526

Change-Id: I92202ae69724f41ce1645af72dbc88e04fdc9f3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107239
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754716}