DNR: Rename rule_indexing_unittest.cc.

Perform the following renames:

  - rule_indexing_unittest.cc -> declarative_net_request_unittest.cc
  - RuleIndexingTestBase -> DeclarativeNetRequestUnittest
  - SingleRulesetIndexingTest -> SingleRulesetTest
  - MultipleRulesetsIndexingTest -> MultipleRulesetsTest

This is necessary because the file and the associated test fixtures now
contain non-indexing related tests. It's now meant to be a generic unit
test class for declarativeNetRequest.

BUG=754526

Change-Id: I71ac1cc1db004b40b0a45f167e43eac97ad6c54e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146290
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759484}

chrome/test/BUILD.gn

@@ -4540,9 +4540,9 @@ test("unit_tests") {
       "../browser/extensions/api/declarative_content/declarative_content_is_bookmarked_condition_tracker_unittest.cc",
       "../browser/extensions/api/declarative_content/declarative_content_page_url_condition_tracker_unittest.cc",
       "../browser/extensions/api/declarative_net_request/action_tracker_unittest.cc",
+      "../browser/extensions/api/declarative_net_request/declarative_net_request_unittest.cc",
       "../browser/extensions/api/declarative_net_request/dnr_test_base.cc",
       "../browser/extensions/api/declarative_net_request/dnr_test_base.h",
-      "../browser/extensions/api/declarative_net_request/rule_indexing_unittest.cc",
       "../browser/extensions/api/declarative_net_request/ruleset_manager_unittest.cc",
       "../browser/extensions/api/declarative_webrequest/webrequest_action_unittest.cc",
       "../browser/extensions/api/declarative_webrequest/webrequest_rules_registry_unittest.cc",