Disable flaking PreferencesTest.MultiProfiles Everywhere

PreferencesTest.MultiProfiles is flaking both on ChromeOS as well as with mash.
This change disables the test everywhere until the flake can be resolved.

This also reverts having the test filtered out for mash.

TBR=jamescook@chromium.org
TEST=PreferencesTest,MultiProfiles

Bug: 787050
Change-Id: Iaa651e1dd23b836f6d8354fbe9e9d2834f855e0b
Reviewed-on: https://chromium-review.googlesource.com/782041


Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518285}