Bug 1396449: Part 2 - Use atoms to test WebExtension permissions. r=krizsa

The extension policy services uses atoms internally for permission names, so
using them directly rather than strings is considerably cheaper.

MozReview-Commit-ID: Io8EuOXHKVy

dom/html/nsHTMLDocument.cpp

@@ -3290,7 +3290,7 @@ nsHTMLDocument::ExecCommand(const nsAString& commandID,
 
   bool restricted = commandID.LowerCaseEqualsLiteral("paste");
   if (restricted && !nsContentUtils::PrincipalHasPermission(&aSubjectPrincipal,
-                                                            NS_LITERAL_STRING("clipboardRead"))) {
+                                                            nsGkAtoms::clipboardRead)) {
     return false;
   }