Initialize the shared memory to appease Valgrind.

tools/valgrind/memcheck/suppressions_mac.txt

 # There are three kinds of suppressions in this file:
 # 1. Third party stuff we have no control over.
 #
 # 2. Intentional unit test errors, 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.
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 2440 matching lines @@
 {
    bug_133076_b
    Memcheck:Uninitialized
    fun:_ZNK7WebCore15DynamicNodeList4nodeEv
    fun:_ZN7WebCore13ChildNodeListD2Ev
    fun:_ZN7WebCore13ChildNodeListD1Ev
    fun:_ZN7WebCore13ChildNodeListD0Ev
    fun:_ZN3WTF10RefCountedIN7WebCore8NodeListEE5derefEv
    fun:_ZN6WebKit11WebNodeList6assignEPN7WebCore8NodeListE
 }
-{
-   bug_138515
-   Memcheck:Uninitialized
-   fun:_ZN5mediaL20InterleaveFloatToIntIsiEEvRKSt6vectorIPfSaIS2_EEPvm
-   fun:_ZN5media20InterleaveFloatToIntERKSt6vectorIPfSaIS1_EEPvmi
-   fun:_ZN11AudioDevice19AudioThreadCallback7ProcessEi
-   fun:_ZN17AudioDeviceThread6Thread3RunEv
-   fun:_ZN17AudioDeviceThread6Thread10ThreadMainEv
-   fun:_ZN4base12_GLOBAL__N_110ThreadFuncEPv
-}