Added addXSLStyleSheet and removeXSLStyleSheet to StyleEngine.

Source/core/dom/DocumentStyleSheetCollection.cpp

Index: Source/core/dom/DocumentStyleSheetCollection.cpp
diff --git a/Source/core/dom/DocumentStyleSheetCollection.cpp b/Source/core/dom/DocumentStyleSheetCollection.cpp
index 2d65470499707cee330edb9c9c4a59597575663a..5d29917d6cd117461a9dc2d740699820241c4d24 100644
--- a/Source/core/dom/DocumentStyleSheetCollection.cpp
+++ b/Source/core/dom/DocumentStyleSheetCollection.cpp
@@ -51,21 +51,7 @@ void DocumentStyleSheetCollection::collectStyleSheetsFromCandidates(StyleEngine*
         Node* n = *it;
         StyleSheetCandidate candidate(*n);
 
-        if (candidate.isXSL()) {
-            // Processing instruction (XML documents only).
-            // We don't support linking to embedded CSS stylesheets, see <https://bugs.webkit.org/show_bug.cgi?id=49281> for discussion.
-            // Don't apply XSL transforms to already transformed documents -- <rdar://problem/4132806>
-            if (RuntimeEnabledFeatures::xsltEnabled() && !document().transformSourceDocument()) {
-                ProcessingInstruction* pi = toProcessingInstruction(n);
-                // Don't apply XSL transforms until loading is finished.
-                if (!document().parsing() && !pi->isLoading())
-                    document().applyXSLTransform(pi);
-                return;
-            }
-
-            continue;
-        }
-
+        ASSERT(!candidate.isXSL());
         if (candidate.isImport()) {
             Document* document = candidate.importedDocument();
             if (!document)