[Signin] Handle Google-Accounts-SignOut for Dice

This CL intercepts the Google-Accounts-Signout header
and implements the signout flow.

Main changes:
- Since multiple accounts can be specified, DiceResponseParams
has now a vector of accounts.
- Since header parameters can be repeated, the parsing function
now uses std::multimap instead of std::map.
- Since header parameters contain whitespace and quotes, additional
sanitization is performed.
- Since this header is not set on redirect,
ProcessDiceResponseHeaderIfExists() is now called on all requests.
- DiceResponseHandler now implements the signout flow.

Bug: 730589
Change-Id: I334e9a884a2d40e8f614870a77c9db0e1cc67cb5
Reviewed-on: https://chromium-review.googlesource.com/548359
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#483691}

• chrome/browser/signin/chrome_signin_helper.cc[diff]
• chrome/browser/signin/dice_response_handler.cc[diff]
• chrome/browser/signin/dice_response_handler.h[diff]
• chrome/browser/signin/dice_response_handler_unittest.cc[diff]
• components/signin/core/browser/dice_header_helper.cc[diff]
• components/signin/core/browser/dice_header_helper.h[diff]
• components/signin/core/browser/fake_signin_manager.cc[diff]
• components/signin/core/browser/fake_signin_manager.h[diff]
• components/signin/core/browser/signin_header_helper.cc[diff]
• components/signin/core/browser/signin_header_helper.h[diff]
• components/signin/core/browser/signin_header_helper_unittest.cc[diff]