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(Reland with WebviewProxyAuthLoginTest browsertext fix)
This CL fixes sign-in behind an authenticated proxy. Two actual changes
were necessary in this context:
- SigninPartitionManager: copy auth data from the system request
  context into new StoragePartitions
  This makes SigninPartitionManager::StartSigninSession async, which
  made small changes necessary in GaiaScreenHandler and
  EnrollmentScreenHandler
- SigninScreenHandler: Don't reload gaia immediately when auth details
  have been supplied. This was unnecessary (the URL Request will
  continue) and actually harmful, as now that we're using a new
  StoragePartition for each sign-in attempt, this discards auth data
  before we can copy them.

To support the browsertest, testserver.py gets an argument to redirect
CONNECT requests to localhost when proxying (in sign-in browsertests
without proxy, we use RuleBasedHostResolverProc::AddRule to achieve
this effect).

      unit_tests --gtest_filter=SigninPartitionManagerTest*
  Manual test:
   Setup:
     Setup an proxy server with Basic authentication
     Configure the device to use the proxy server.
     Make sure the user adding screen is shown.
   Test 1:
     Expect proxy auth dialog. Enter correct proxy auth data.
     Expect that sign-in screen is shown and sign-in works.
   Test 2:
     Expect proxy auth dialog. Enter incorrect proxy auth data.
     Expect that proxy-auth dialog is shown again.
   Test 3:
     Expect proxy auth dialog. Press Cancel.
     Expect that a network error dialog is shown.
     Press "try to sign in again".
     Expect proxy auth dialog. Enter correct proxy auth data.
     Expect that sign-in screen is shown and sign-in works.

Bug: 793524
Test: browser_tests --gtest_filter=WebviewProxyAuthLoginTest.* &&
Change-Id: I56dafa240ad3bb5902517688a5cb17e309f2982d
Reviewed-on: https://chromium-review.googlesource.com/850472Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#527103}