Update //third_party/subresource-filter-ruleset for WebLayer use

WebLayer plans to use the ruleset in this directory. We therefore update
README.chromium to describe this use. We also add manifest.json, which
is currently distributed by the component updater, to allow WebLayer to
easily determine the filter list version number.

Bug: 1139369
Change-Id: Ib4b5a1ff086918148c3b15917524a92094677427
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495603Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Alex Turner <alexmt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823776}

third_party/subresource-filter-ruleset/README.chromium

@@ -2,15 +2,20 @@ Name: EasyList
 URL: https://easylist.to/easylist/easylist.txt
 Version: 202010221614
 License: Creative Commons Attribution-ShareAlike 3.0 Unported
-License File: NOT_SHIPPED
+License File: LICENSE
 Security Critical: no
 
 Description:
 After fetching from Google storage, the data/ folder contains a filter list
 derived from EasyList. It is used for the Subresource Filter, as described at
-https://cs.chromium.org/chromium/src/components/subresource_filter/README.md
+https://cs.chromium.org/chromium/src/components/subresource_filter/README.md.
+This copy is included in WebLayer releases, but is not used for Chrome. Instead,
+Chrome's filter list is updated via the component updater.
 
 Local Modifications:
 The filter list is processed to reduce its size substantially. We include the
 unindexed version of this smaller ruleset. The full process is described at
-https://cs.chromium.org/chromium/src/components/subresource_filter/FILTER_LIST_GENERATION.md
+https://cs.chromium.org/chromium/src/components/subresource_filter/FILTER_LIST_GENERATION.md.
+A manifest.json file is added to the top-level directory and is not based on
+EasyList. It mirrors the manifest.json file distributed by component updater for
+Chrome and lists version information for the derived list.

third_party/subresource-filter-ruleset/manifest.json

+{
+  "manifest_version": 2,
+  "name": "Subresource Filtering Rules",
+  "ruleset_format": 1,
+  "version": "9.17.0"
+}