Add Schema.org schema file for eventual use in document metadata extractor

Bug: 1044249
Change-Id: I163919382fd8d40a2355a8da71d72db68018e131
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023864Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Sam Bowen <sgbowen@google.com>
Cr-Commit-Position: refs/heads/master@{#738267}

third_party/schema_org/OWNERS

+beccahughes@chromium.org
+sgbowen@google.com
+steimel@chromium.org
+
+# COMPONENT: Blink>Media>Session
+# TEAM: media-dev@chromium.org

third_party/schema_org/README.chromium

+Name: Schema.org is a collaborative, community activity with a mission to
+create, maintain, and promote schemas for structured data on the Internet.
+Short Name: schema_org
+URL: http://schema.org/version/6.0/schema.jsonld 
+Version: 6.0
+Date: 20200121
+License: Creative Commons Attribution-ShareAlike 3.0 Unported
+License File: LICENSE
+Security Critical: yes
+
+Description:
+A script uses the schema file in this directory to generate C++ object code.
+Using the generated code, Blink extracts schema.org metadata from the page. This
+metadata is used for a variety of purposes. See the document_metadata module in
+Blink:
+https://cs.chromium.org/chromium/src/third_party/blink/renderer/modules/document_metadata/
+
+Local Modifications:
+None.