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

BUG=662005

Review-Url: https://codereview.chromium.org/2574303002
Cr-Commit-Position: refs/heads/master@{#438869}

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;
 };

third_party/WebKit/Source/modules/imagecapture/ImageCapture.idl

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://www.w3.org/TR/image-capture
+// https://w3c.github.io/mediacapture-image/#ImageCaptureAPI
 
 [
     ActiveScriptWrappable,

third_party/WebKit/Source/modules/presentation/PresentationConnection.idl

@@ -13,7 +13,7 @@ enum PresentationConnectionState {
 [
     RuntimeEnabled=Presentation
 ] interface PresentationConnection : EventTarget {
-    readonly attribute DOMString? id;
+    readonly attribute DOMString id;
     readonly attribute PresentationConnectionState state;
     [MeasureAs=PresentationConnectionClose] void close();
     [MeasureAs=PresentationConnectionTerminate] void terminate();

third_party/WebKit/Source/modules/push_messaging/PushEvent.idl

@@ -9,5 +9,5 @@
     Exposed=ServiceWorker,
     RuntimeEnabled=PushMessaging,
 ] interface PushEvent : ExtendableEvent {
-    readonly attribute PushMessageData data;
+    readonly attribute PushMessageData? data;
 };