Valgrind: Add a new mac suppression and update an existing one.

tools/valgrind/memcheck/suppressions.txt

 # There are four kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 1027 matching lines @@
    bug_9503
    Memcheck:Leak
    ...
    fun:_ZN19TestWebViewDelegate24UpdateSelectionClipboardEb
 }
 {
    # See http://crbug.com/11139
    bug_11139
    Memcheck:Leak
    fun:_Znw*
-   fun:_ZN14ProcessWatcher23EnsureProcessTerminatedEi
+   fun:_ZN4base23EnsureProcessTerminatedEi
 }
 {
    # very common in ui tests
    bug_16089a
    Memcheck:Leak
    ...
    fun:_ZN4base22PosixDynamicThreadPool8PostTask*
    ...
    fun:_ZN3net12HostResolver3Job5StartEv
 }
@@ skipping 4238 matching lines @@
    obj:/lib/libselinux.so.1
    obj:/lib/libselinux.so.1
 }
 {
    bug_todo_grep
    Memcheck:Leak
    fun:malloc
    obj:/bin/grep
    obj:/bin/grep
 }