Bug 1425759: Simplify the style backend type setup. r=bholley

Now that what we use to decide whether a document is styled by Servo are only
prefs and the doc principal, we don't need to inherit the style backend type,
since unless the pref has changed, the result will be the same.

MozReview-Commit-ID: KBmeBn1cRne

dom/xhr/XMLHttpRequestMainThread.cpp

@@ -200,7 +200,6 @@ XMLHttpRequestMainThread::XMLHttpRequestMainThread()
     mResponseType(XMLHttpRequestResponseType::_empty),
     mRequestObserver(nullptr),
     mState(XMLHttpRequestBinding::UNSENT),
-    mStyleBackend(StyleBackendType::None),
     mFlagSynchronous(false), mFlagAborted(false), mFlagParseBody(false),
     mFlagSyncLooping(false), mFlagBackgroundRequest(false),
     mFlagHadUploadListenersOnSend(false), mFlagACwithCredentials(false),
@@ -2024,8 +2023,7 @@ XMLHttpRequestMainThread::OnStartRequest(nsIRequest *request, nsISupports *ctxt)
                            emptyStr, emptyStr, nullptr, docURI,
                            baseURI, requestingPrincipal, true, global,
                            mIsHtml ? DocumentFlavorHTML :
-                                     DocumentFlavorLegacyGuess,
-                           mStyleBackend);
+                                     DocumentFlavorLegacyGuess);
     NS_ENSURE_SUCCESS(rv, rv);
     mResponseXML = do_QueryInterface(responseDoc);
     mResponseXML->SetChromeXHRDocURI(chromeXHRDocURI);