Reland: Support multiple sign-in for extension updates.

Reland: Support multiple sign-in for extension updates.

This changes the ExtensionDownloader to attempt multiple
sequential queries for CRX downloads, iterating over incremental
values for the authuser query parameter up to a max of 10.

This is a re-landing of http://codereview.chromium.org/279453002/
which was reverted due to a use-after-free bug in the unit tests.

BUG=370964
R=asargent@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269666

[Ken Rockot(use gerrit already), 2014-05-09 22:30:45]

Note that this is the same patch as before, except the unit tests now reacquire
a potentially new TestURLFetcher after every runloop cycle, since the fetcher
may be freed by ExtensionDownloader.

[asargent_no_longer_on_chrome, 2014-05-09 23:22:32]

lgtm

[Ken Rockot(use gerrit already), 2014-05-09 23:26:49]

The CQ bit was checked by rockot@chromium.org

[commit-bot: I haz the power, 2014-05-09 23:31:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/rockot@chromium.org/274263002/1

[commit-bot: I haz the power, 2014-05-10 03:42:12]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  win_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-05-11 01:07:31]

Change committed as 269666