Bug 1295082 - Put Extension in BaseContext r=kmag

ExtensionContext in Extension.jsm has |extension| as an instance member,
so use it instead of passing |extension| to registerSchemaAPI's
callback.

And to make sure that this pattern also works in content processes, move
the |extension| member to BaseContext.

MozReview-Commit-ID: BgsGGCPQxJR

browser/components/extensions/ext-commands.js

@@ -228,7 +228,8 @@ extensions.on("shutdown", (type, extension) => {
 });
 /* eslint-enable mozilla/balanced-listeners */
 
-extensions.registerSchemaAPI("commands", (extension, context) => {
+extensions.registerSchemaAPI("commands", context => {
+  let {extension} = context;
   return {
     commands: {
       getAll() {