GitLab

This patch does three related things:

1. Plumbs an optional IdentityProvider through 
ExtensionUpdater / ExtensionDownloader.

2. Adds support in ExtensionDownloader for OAuth2 token-
based authentication to protected Webstore download URLs,
using the optional IdentityProvider as its account ID and 
token source. Bonus points for general cleanup of the 
credential iteration code in ExtensionDownloader.

3. Wires up Chrome's ExtensionDownloader (via
ExtensionService) with a ProfileIdentityProvider so that
it may authenticate as the Chrome-signed-in user via OAuth2.

BUG=376553,122763

Review URL: https://codereview.chromium.org/434493002

Cr-Commit-Position: refs/heads/master@{#288261}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288261 0039d316-1c4b-4281-b951-d872f2087c98