Bug 1731526 - Fix non-unified build errors in accessible/. r=eeejay

accessible/base/TreeWalker.cpp:151:10: error: 'return' will never be executed [-Werror,-Wunreachable-code-return]
accessible/xpcom/xpcAccessibleMacInterface.mm:177:10: error: 'return' will never be executed [-Werror,-Wunreachable-code-return]

Differential Revision: https://phabricator.services.mozilla.com/D126173

accessible/base/TreeWalker.cpp

@@ -148,7 +148,7 @@ bool TreeWalker::Seek(nsIContent* aChildNode) {
     }
   } while (true);
 
-  return false;
+  MOZ_ASSERT_UNREACHABLE("because the do-while loop never breaks");
 }
 
 LocalAccessible* TreeWalker::Next() {

accessible/xpcom/xpcAccessibleMacInterface.mm

@@ -173,8 +173,6 @@ xpcAccessibleMacInterface::GetParameterizedAttributeValue(const nsAString& aAttr
   NSString* attribName = nsCocoaUtils::ToNSString(aAttributeName);
   return NSObjectToJsValue(
       [mNativeObject accessibilityAttributeValue:attribName forParameter:paramObj], aCx, aResult);
-
-  return NS_OK;
 }
 
 bool xpcAccessibleMacInterface::SupportsSelector(SEL aSelector) {