GitLab

cros: WallpaperControllerTest should use ScopedTempDir

The flakiness is not related to RunUntilIdle(), but is because
ScopedTempDir is not used for custom or online wallpapers: the tests
do lots of operations on the directories, so we should make sure they
are unique to avoid affecting each other.

Bug: 823760, 823762
Test: ash_unittests (Run all tests together)
Change-Id: I9cf6a70a4cfc146c8a0b91ed4cf2860dbc72394f
Reviewed-on: https://chromium-review.googlesource.com/1252981
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595308}