base: Fix MemoryPressureListener documentation

ObserverListThreadSafe will call PostTask on all observer callbacks,
even those on the same thread. So correct the comment in
MemoryPressureListener which states callbacks on the same thread as the
system broadcast are called synchronously.

BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/25676002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226497 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226497