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The first fix is to get the CookieManager from the render process' StoragePartition, instead of
using GetDefaultStoragePartition. The second fix is to not use the StoragePartition's CookieManager
(which is used for web requests) when ContentBrowserClient::OverrideRequestContextForURL returns a
different store (e.g. for chrome-extension scheme).

Bug: 769401

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: Id61abda2cd38957867fb3848d06b1b18d9e89b79
Reviewed-on: https://chromium-review.googlesource.com/957554Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543186}