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

base/memory/memory_pressure_listener.h

@@ -24,11 +24,8 @@ namespace base {
 // simply delete the listener object. The implementation guarantees
 // that the callback will always be called on the thread that created
 // the listener.
-// If this is the same thread as the system is broadcasting the memory pressure
-// event on, then it is guaranteed you're called synchronously within that
-// broadcast and hence you should not do long-running garbage collection work.
-// But conversely, if there's something that needs to be released before
-// control is returned to system code, this is the place to do it.
+// Note that even on the same thread, the callback is not guaranteed to be
+// called synchronously within the system memory pressure broadcast.
 // Please see notes on memory_pressure_level_list.h: some levels are absolutely
 // critical, and if not enough memory is returned to the system, it'll
 // potentially kill the app, and then later the app will have to be