Add a referrer parameter to the app details fetcher.
This is the first part of a three-sided patch which will allow
developers to specify a referrer in the URL of a manifest related
application. The referrer will then be added to the app details
fetch and sent to the relevant store (currently just the Play Store).
The outdated method and dummy implementations will be removed in the
third side of this patch once the relevant downstream method is updated.
BUG=503226
Review URL: https://codereview.chromium.org/
1277743003
Cr-Commit-Position: refs/heads/master@{#342236}