Use a valid WebMouseWheelEvent in stringification test

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 # running overnight and removing outdated bugs/suppressions.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 3452 matching lines @@
    Memcheck:Uninitialized
    fun:_ZNK5blink20InlineFlowBoxPainter49roundedFrameRectClampedToLineTopAndBottomIfNeededEv
    fun:_ZN5blink20InlineFlowBoxPainter28paintBoxDecorationBackgroundERNS_9PaintInfoERKNS_11LayoutPointE
    fun:_ZN5blink20InlineFlowBoxPainter5paintERNS_9PaintInfoERKNS_11LayoutPointENS_10LayoutUnitES6_
    fun:_ZN5blink13InlineFlowBox5paintERNS_9PaintInfoERKNS_11LayoutPointENS_10LayoutUnitES6_
    fun:_ZN5blink20InlineFlowBoxPainter5paintERNS_9PaintInfoERKNS_11LayoutPointENS_10LayoutUnitES6_
    fun:_ZN5blink13InlineFlowBox5paintERNS_9PaintInfoERKNS_11LayoutPointENS_10LayoutUnitES6_
    fun:_ZN5blink13RootInlineBox5paintERNS_9PaintInfoERKNS_11LayoutPointENS_10LayoutUnitES6_
 }
 {
-   bug_422083
-   Memcheck:Uninitialized
-   ...
-   fun:_ZN7content12_GLOBAL__N_137WebInputEventTraitsTest_ToString_Test8TestBodyEv
-}
-{
    bug_422087a
    Memcheck:Leak
    fun:_Znw*
    fun:_ZN4base8internal20PostTaskAndReplyImpl16PostTaskAndReplyERKN15tracked_objects8LocationERKNS_8CallbackIFvvEEESA_
    fun:_ZN4base10TaskRunner16PostTaskAndReplyERKN15tracked_objects8LocationERKNS_8CallbackIFvvEEES9_
    fun:_ZN7content13PpFrameWriter19FrameWriterDelegate12DeliverFrameERK13scoped_*
 }
 {
    bug_422087b
    Memcheck:Leak
    fun:_Znw*
    ...
    fun:_ZNSt4listI13scoped_refptrIN5media10VideoFrameEESaIS3_EE9push_backERKS3_
    fun:_ZN5media14VideoFramePool8PoolImpl13FrameReleasedERK13scoped_refptrINS_10VideoFrameEE
 }