Bug 1390951 - Use the same style backend when cloning documents. r=xidorn

MozReview-Commit-ID: F6HMAsSsNDa

dom/base/nsDocument.cpp

@@ -9607,6 +9607,7 @@ nsDocument::CloneDocHelper(nsDocument* clone, bool aPreallocateChildren) const
   clone->mContentLanguage = mContentLanguage;
   clone->SetContentTypeInternal(GetContentTypeInternal());
   clone->mSecurityInfo = mSecurityInfo;
+  clone->mStyleBackendType = mStyleBackendType;
 
   // State from nsDocument
   clone->mType = mType;