Create an empty StyleSheetList for moreStyleSheets.

We created a StyleSheetList connected to the TreeScope which would
create a second StyleSheetList for the scope-attached sheets which
messed up the active stylesheets causing crashes.

Bug: 819483
Change-Id: Ib24bee538a27a64b5a8206e32334d4324629d5af
Reviewed-on: https://chromium-review.googlesource.com/964326
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543654}

third_party/WebKit/LayoutTests/fast/css/CSSStyleSheet-constructable.html

@@ -2,6 +2,7 @@
 <!-- TODO(rakina): move to WPT once spec is finalized -->
 <script src = '../../resources/testharness.js'></script>
 <script src = '../../resources/testharnessreport.js'></script>
+<style>dummy {}</style>
 
 <section id="firstSection">
   <div>
@@ -38,6 +39,10 @@ const blueShadowSpan = secondDiv.children[2];
 const whiteShadowSpan = secondDiv.children[3];
 const yellowShadowSpan = secondDiv.children[4];
 
+test(() => {
+  assert_equals(document.moreStyleSheets.length, 0);
+}, "document.moreStyleSheets should initially have length 0.");
+
 test(() => {
   // Style sheets can be created
   const greenStyleSheet = new CSSStyleSheet(greenStyleText);
@@ -283,4 +288,3 @@ test(() => {
   assert_equals(getComputedStyle(yellowIframeSpan).color, "rgb(0, 0, 0)");
 }, 'Constructed stylesheet can be used and modified in multiple Documents');
 </script>
-

third_party/WebKit/Source/core/css/StyleSheetList.cpp

@@ -31,6 +31,11 @@ namespace blink {
 
 using namespace HTMLNames;
 
+StyleSheetList* StyleSheetList::Create() {
+  DCHECK(RuntimeEnabledFeatures::ConstructableStylesheetsEnabled());
+  return new StyleSheetList();
+}
+
 StyleSheetList* StyleSheetList::Create(
     const HeapVector<Member<CSSStyleSheet>>& style_sheet_vector,
     ExceptionState& exception_state) {
@@ -38,7 +43,6 @@ StyleSheetList* StyleSheetList::Create(
     exception_state.ThrowTypeError("Illegal constructor");
     return nullptr;
   }
-
   return new StyleSheetList(style_sheet_vector);
 }
 

third_party/WebKit/Source/core/css/StyleSheetList.h

@@ -37,6 +37,7 @@ class CORE_EXPORT StyleSheetList final : public ScriptWrappable {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
+  static StyleSheetList* Create();
   static StyleSheetList* Create(const HeapVector<Member<CSSStyleSheet>>&,
                                 ExceptionState&);
 
@@ -60,6 +61,7 @@ class CORE_EXPORT StyleSheetList final : public ScriptWrappable {
  private:
   explicit StyleSheetList(const HeapVector<Member<CSSStyleSheet>>&);
   explicit StyleSheetList(TreeScope*);
+  StyleSheetList() {}
   const HeapVector<TraceWrapperMember<StyleSheet>>& StyleSheets() const;
 
   Member<TreeScope> tree_scope_;

third_party/WebKit/Source/core/dom/TreeScope.cpp

@@ -327,7 +327,7 @@ bool TreeScope::HasMoreStyleSheets() const {
 
 StyleSheetList& TreeScope::MoreStyleSheets() {
   if (!more_style_sheets_)
-    SetMoreStyleSheets(StyleSheetList::Create(this));
+    SetMoreStyleSheets(StyleSheetList::Create());
   return *more_style_sheets_;
 }