Don't trigger full active style update on styleSheets access.

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

Index: third_party/WebKit/Source/core/dom/StyleSheetCollection.cpp
diff --git a/third_party/WebKit/Source/core/dom/StyleSheetCollection.cpp b/third_party/WebKit/Source/core/dom/StyleSheetCollection.cpp
index bd7a515a8c719f868d9776185067185b229a8f41..286777389f724c53aeea8bfd21ba9cec05fb17ba 100644
--- a/third_party/WebKit/Source/core/dom/StyleSheetCollection.cpp
+++ b/third_party/WebKit/Source/core/dom/StyleSheetCollection.cpp
@@ -44,13 +44,13 @@ void StyleSheetCollection::Swap(StyleSheetCollection& other) {
   ::blink::swap(style_sheets_for_style_sheet_list_,
                 other.style_sheets_for_style_sheet_list_, this, &other);
   active_author_style_sheets_.swap(other.active_author_style_sheets_);
+  sheet_list_dirty_ = false;
 }
 
 void StyleSheetCollection::SwapSheetsForSheetList(
     HeapVector<Member<StyleSheet>>& sheets) {
-  // Only called for collection of HTML Imports that never has active sheets.
-  DCHECK(active_author_style_sheets_.IsEmpty());
   ::blink::swap(style_sheets_for_style_sheet_list_, sheets, this);
+  sheet_list_dirty_ = false;
 }
 
 void StyleSheetCollection::AppendActiveStyleSheet(