Use STLValueDeleter for maps in ProfileSyncServicePreferenceTest.

These tests have maps with pointer values, prefer STLValueDeleter next
to the definition of the map over STLDeleteValues at the end of the
test to make it slightly harder to lop off the cleanup code.

BUG=299290
TEST=unit_test.exe

Review URL: https://codereview.chromium.org/32363005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230045 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230045