[Extensions] Update PrefHashBrowserTestBase to manifest v2

PrefHashBrowserTestBase tests have been updated to manifest v2.
Remove the allowance for manifest v1 extensions.

Bug: 816679

Change-Id: Idfa6c27efac318842ad5169370e2073d02edb615
Reviewed-on: https://chromium-review.googlesource.com/1014561
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553313}
1 file changed