Fix use-of-uninitialized-value in AXNodeObject::AlterSliderOrSpinButtonValue

Bug: 1057369
Change-Id: Ie17d277b09c084eb224cb3228deb489d2ce96168
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083434
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746086}

third_party/blink/renderer/modules/accessibility/ax_node_object.cc

@@ -125,7 +125,8 @@ void AXNodeObject::AlterSliderOrSpinButtonValue(bool increase) {
     return;
 
   float step;
-  StepValueForRange(&step);
+  if (!StepValueForRange(&step))
+    return;
 
   value += increase ? step : -step;