Adds README.chromium for chrome://credits page generation

Bug: 884663
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I0075911b60df18873c8c64db17d3455d567e9ea6
Reviewed-on: https://chromium-review.googlesource.com/c/1236661Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601171}

ios/third_party/firebase/LICENSE

+GOOGLE ANALYTICS FOR FIREBASE TERMS OF SERVICE
+Terms last modified: Oct 1, 2018
+
+Excerpt from: https://firebase.google.com/terms/analytics/
+
+4. Nonexclusive License.
+
+Subject to the terms and conditions of this Agreement, (a) Google
+grants You a limited, revocable, non-exclusive, non-sublicensable
+license to install, copy and use the SDK solely as necessary for
+You to use the Service on Your Apps or Third Parties Apps; and (b)
+You may remotely access, view and download Your Reports. You will
+not (and You will not allow any third party to) use data labeled
+as belonging to a third party in the Service for purposes other
+than generating, viewing, and downloading Reports. You will comply
+with all applicable laws and regulations and Your agreements with
+third parties in Your use of and access to the Documentation,
+Software, Service and Reports.

ios/third_party/firebase/README.chromium

+Name: Firebase SDK
+Short Name: Firebase
+URL: https://firebase.google.com/terms/
+Version: 5.8.0
+License: Custom License
+License File: LICENSE
+
+Description:
+This directory contains Analytics package from Firebase SDK.
+The portion of Firebase SDK used by Google Chrome for iOS
+is extracted and repacked into a CIPD packaged.
+See cipd.yaml for additional info.