Do not allow values to be migrated without MACs if the destination already has a MAC.

Do not allow values to be migrated without MACs if the destination already has a MAC.

BUG=414554
TEST=Added regression test PrefHashBrowserTestUntrustedAdditionToPrefs which fails without this CL and passes with it.

Review URL: https://codereview.chromium.org/550343004

Cr-Commit-Position: refs/heads/master@{#295048}
(cherry picked from commit b004a56cd18f7519d1cceafb534a5865f93d988d)

Conflicts:
	(DNE in 37; test running on trunk and beta-builders is sufficient) chrome/browser/prefs/tracked/pref_hash_browsertest.cc

TBR=gab@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/39f8a1461d36f9f8f749bf59071a8a194f6569e2

[gab, 2014-09-17 15:13:33]

gab@chromium.org changed reviewers:
+ gab@chromium.org

[gab, 2014-09-18 00:56:45]

Committed patchset #1 (id:1) manually as 39f8a14 (presubmit successful).