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This allows setting command-line flags before AshTestHelper is
constructed.

Bug: none
Change-Id: Iab006b3b144948f7a982a6b591dc5e4527b2feb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125179Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754522}