[MemSheriff] Suppress libpng uninit report

BUG=396658
TBR=earthdok@chromium.org

Review URL: https://codereview.chromium.org/417573003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284950 0039d316-1c4b-4281-b951-d872f2087c98

tools/valgrind/memcheck/suppressions.txt

@@ -6212,3 +6212,10 @@
    fun:_ZN7content28ShellURLRequestContextGetter20GetURLRequestContextEv
    fun:_ZN7content21ChromeAppCacheService20InitializeOnIOThreadERKN4base8FilePathEPNS_15ResourceContextEPN3net23URLRequestContextGetterE13scoped_refptrIN5quota20SpecialStoragePolicyEE
 }
+{
+   bug_396658
+   Memcheck:Uninitialized	
+   ...
+   fun:wk_png_write_find_filter	
+   fun:wk_png_write_row	
+}