Add leak suppression for FirstRunMasterPrefs test under DrMemory.

TBR=oshima@chromium.org
BUG=487500

Review URL: https://codereview.chromium.org/1141693002

Cr-Commit-Position: refs/heads/master@{#329587}

tools/valgrind/drmemory/suppressions.txt

@@ -777,3 +777,12 @@ blink_web.dll!blink::WebFrameWidgetImpl::setIsAcceleratedCompositingActive
 blink_web.dll!blink::WebFrameWidgetImpl::setRootGraphicsLayer
 blink_web.dll!blink::ChromeClientImpl::attachRootGraphicsLayer
 blink_web.dll!blink::DeprecatedPaintLayerCompositor::detachRootLayer
+
+HANDLE LEAK
+name=http://crbug.com/487500
+system call NtCreateEvent
+...
+v8.dll!v8::base::Mutex::Lock
+base.dll!base::`anonymous namespace'::WorkItemCallback
+ntdll.dll!vsnprintf
+ntdll.dll!TpCallbackIndependent