Fix low risk non-nullable => nullable attributes / arguments to match the spec

third_party/WebKit/Source/core/dom/ProcessingInstruction.idl

Index: third_party/WebKit/Source/core/dom/ProcessingInstruction.idl
diff --git a/third_party/WebKit/Source/core/dom/ProcessingInstruction.idl b/third_party/WebKit/Source/core/dom/ProcessingInstruction.idl
index c0d0f67c2765c59e4abb9075fff6b880ad175d81..353ad410adc86c797cdcd4bf4c52c12a621d3ca0 100644
--- a/third_party/WebKit/Source/core/dom/ProcessingInstruction.idl
+++ b/third_party/WebKit/Source/core/dom/ProcessingInstruction.idl
@@ -25,5 +25,5 @@ interface ProcessingInstruction : CharacterData {
 
     // ProcessingInstruction implements LinkStyle
     // https://dev.w3.org/csswg/cssom/#requirements-on-user-agents-implementing-the-xml-stylesheet-processing-instruction
-    readonly attribute StyleSheet sheet;
+    readonly attribute StyleSheet? sheet;
 };