Make manifest URL prefix configurable.
Add a manifest_url_prefix parameter to PrecacheFetcher. If non-empty, its value
will override the flag-specified default. Change PrecacheManager to fetch the
value of the "manifest_url_prefix" variation parameter for the Precache field
trial (empty string if not present), and pass it to PrecacheFetcher.
This way, we can easily switch between multiple versions of the
manifests, and even experiment on multiple variants simultaneously.
BUG=309216
Review URL: https://codereview.chromium.org/
1177983002
Cr-Commit-Position: refs/heads/master@{#334765}