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See https://crbug.com/879348 for a description of the issue. This CL
adds a DownloadPrefs::ReinitializeDefaultDownloadDirectoryForTesting()
method to force the global DefaultDownloadDirectory instance to
reinitialize itself to take into account any test configurations.

Bug: 879348
Change-Id: I4a2340fad5f1396196e6746073857d3a09bfa7c1
Reviewed-on: https://chromium-review.googlesource.com/1197724Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589291}