Avoid unchecked casts in UA shadow DOM.

third_party/WebKit/Source/core/html/forms/RangeInputType.cpp

Index: third_party/WebKit/Source/core/html/forms/RangeInputType.cpp
diff --git a/third_party/WebKit/Source/core/html/forms/RangeInputType.cpp b/third_party/WebKit/Source/core/html/forms/RangeInputType.cpp
index 5a46a2257b62582efb9604dbf2c305573bf72364..926f555cbfabde126fa139bac1ead9a12431de14 100644
--- a/third_party/WebKit/Source/core/html/forms/RangeInputType.cpp
+++ b/third_party/WebKit/Source/core/html/forms/RangeInputType.cpp
@@ -322,8 +322,9 @@ bool RangeInputType::shouldRespectListAttribute() {
 }
 
 inline SliderThumbElement* RangeInputType::sliderThumbElement() const {
-  return toSliderThumbElement(element().userAgentShadowRoot()->getElementById(
-      ShadowElementNames::sliderThumb()));
+  return toSliderThumbElementOrDie(
+      element().userAgentShadowRoot()->getElementById(
+          ShadowElementNames::sliderThumb()));
 }
 
 inline Element* RangeInputType::sliderTrackElement() const {