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|InlineLoginHandlerChromeOS| and |ChromeOSOAuth2TokenServiceDelegate|
use the system URL context because they 'know' that AccountManager is
going to use the same. This is a leaky abstraction.

Add a method in AccountManager that returns its URL Request Context.
This request context happens to be the system URL context for now.

Bug: 820046
Change-Id: I668769167893c995f73df63acc575eb410c0f986
Reviewed-on: https://chromium-review.googlesource.com/1088617
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566998}