Suppress WebCore::ThreadState::visitStack uninits when it walks the stack

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 # running overnight and removing outdated bugs/suppressions.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 6328 matching lines @@
    fun:_ZN7WebCore15DOMWrapperWorld9mainWorldEv
    fun:_ZN7WebCore16ScriptControllerC2EPNS_5FrameE
    fun:_ZN7WebCore16ScriptControllerC1EPNS_5FrameE
    fun:_ZN7WebCore5FrameC2EN3WTF10PassRefPtrINS_9FrameInitEEE
    fun:_ZN7WebCore5FrameC1EN3WTF10PassRefPtrINS_9FrameInitEEE
    fun:_ZN7WebCore5Frame6createEN3WTF10PassRefPtrINS_9FrameInitEEE
    fun:_ZN5blink12WebFrameImpl21initializeAsMainFrameEPN7WebCore4PageE
    fun:_ZN5blink11WebViewImpl12setMainFrameEPNS_8WebFrameE
    fun:_ZN7content22BufferedDataSourceTestC2Ev
 }
+{
+   bug_340752
+   Memcheck:Uninitialized
+   ...
+   fun:_ZN7WebCore17HeapContainsCache*
+   fun:_ZN7WebCore11ThreadState19heapPageFromAddressEPh
+   fun:_ZN7WebCore11ThreadState19checkAndMarkPointerEPNS_7VisitorEPh
+   fun:_ZN7WebCore4Heap19checkAndMarkPointerEPNS_7VisitorEPh
+   fun:_ZN7WebCore11ThreadState10visitStackEPNS_7VisitorE
+   fun:_ZN7WebCore11ThreadState5traceEPNS_7VisitorE
+   fun:_ZN7WebCore11ThreadState10visitRootsEPNS_7VisitorE
+   fun:_ZN7WebCore4Heap14collectGarbageENS_11ThreadState10StackStateENS0_6GCTypeE
+}