Clear app's SharedPreferences before each test to reduce flakiness.

It should fix FirstRun tests flakiness issue if FirstRun tests were
failed because shared pref being set by a previous test.

Bug: 908174, 902774
Change-Id: I9480750a60da1344a72e105ffc444dd982cb36b1
Reviewed-on: https://chromium-review.googlesource.com/c/1399542
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Commit-Queue: Yun Liu <yliuyliu@google.com>
Cr-Commit-Position: refs/heads/master@{#622569}
1 file changed