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Relanding of https://codereview.chromium.org/465543004
with a fix for official builds.

This establishes a ManifestFetchDataDelegate for use
by ExtensionDownloader and ManifestFetchData, and moves
ManifestFetchData into //extensions/browser/updater.

The delegate provides implementation details for
update manifest fetching, including brand code,
boilerplate query parameters, and ping data.

Chrome's implementation has knowledge of the
Google brand, the metrics service, and Omaha-specific
query parameters necessary for CRX item queries.

BUG=398671
TBR=isherman@chromium.org

Review URL: https://codereview.chromium.org/475423002

Cr-Commit-Position: refs/heads/master@{#295142}