Issue 5300 - Add feature detection for object_subrequest

lib/requestBlocker.js

Index: lib/requestBlocker.js
===================================================================
--- a/lib/requestBlocker.js
+++ b/lib/requestBlocker.js
@@ -24,19 +24,19 @@
 const {defaultMatcher} = require("matcher");
 const {FilterNotifier} = require("filterNotifier");
 const {Prefs} = require("prefs");
 const {checkWhitelisted, getKey} = require("whitelisting");
 const {stringifyURL, extractHostFromFrame, isThirdParty} = require("url");
 const {port} = require("messaging");
 const devtools = require("devtools");
 
-// Chrome and Firefox (WebExtensions) can't distinguish between
-// OBJECT_SUBREQUEST and OBJECT requests.
-RegExpFilter.typeMap.OBJECT_SUBREQUEST = RegExpFilter.typeMap.OBJECT;
+// Chrome can't distinguish between OBJECT_SUBREQUEST and OBJECT requests.
+if (!("OBJECT_SUBREQUEST" in chrome.webRequest.ResourceType))
+  RegExpFilter.typeMap.OBJECT_SUBREQUEST = RegExpFilter.typeMap.OBJECT;
 
 // Map of content types reported by the browser to the respecitve content types
 // used by Adblock Plus. Other content types are simply mapped to OTHER.
 let resourceTypes = new Map(function*()
 {
   for (let type in RegExpFilter.typeMap)
     yield [type.toLowerCase(), type];