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Currently we create default web request rules registries for the declarative web
request API even if the API is not available to the current environment. This is
wasteful and detrimental to performance, since the initial registry load blocks
the first network request (tracked via Extensions.NetworkDelayRegistryLoad UMA).

This CL changes RulesRegistryService so that the default WebRequestRulesRegistry
is registered only if the declarative web request API is avaialble to the
current environment. This, for example, means that no default web request rule
registries would be created on the stable channel. Rules registries
corresponding to webviews and the default content rules registry would still be
created.

This also helps fix issue 777717 and renderer cache is not cleared redundantly
on each extension load/unload/uninstall when the API is not available.

BUG=693243, 777717

Change-Id: I7384fed71a86aea3f5cc8d2eafd1445b439dd76a
Reviewed-on: https://chromium-review.googlesource.com/1072497Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563051}