Don't try to set the empty string as the preferred set.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2011, 2012 Apple Inc. All
  * rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved.
  * (http://www.torchmobile.com/)
  * Copyright (C) 2008, 2009, 2011, 2012 Google Inc. All rights reserved.
@@ skipping 542 matching lines @@
     DCHECK(contents);
     DCHECK(contents->IsCacheableForStyleElement());
     DCHECK(contents->HasSingleOwnerDocument());
     contents->SetIsUsedFromTextCache();
     style_sheet =
         CSSStyleSheet::CreateInline(contents, element, start_position);
   }
 
   DCHECK(style_sheet);
   if (!element.IsInShadowTree()) {
-    style_sheet->SetTitle(element.title());
-    SetPreferredStylesheetSetNameIfNotSet(element.title());
+    String title = element.title();
+    if (!title.IsEmpty()) {
+      style_sheet->SetTitle(title);
+      SetPreferredStylesheetSetNameIfNotSet(title);
+    }
   }
   return style_sheet;
 }
 
 CSSStyleSheet* StyleEngine::ParseSheet(Element& element,
                                        const String& text,
                                        TextPosition start_position) {
   CSSStyleSheet* style_sheet = nullptr;
   style_sheet = CSSStyleSheet::CreateInline(element, KURL(), start_position,
                                             GetDocument().characterSet());
@@ skipping 342 matching lines @@
     style_resolver_stats_ = nullptr;
     return;
   }
   if (!style_resolver_stats_)
     style_resolver_stats_ = StyleResolverStats::Create();
   else
     style_resolver_stats_->Reset();
 }
 
 void StyleEngine::SetPreferredStylesheetSetNameIfNotSet(const String& name) {
+  DCHECK(!name.IsEmpty());
   if (!preferred_stylesheet_set_name_.IsEmpty())
     return;
   preferred_stylesheet_set_name_ = name;
   // TODO(rune@opera.com): Setting the selected set here is wrong if the set
   // has been previously set by through Document.selectedStylesheetSet. Our
   // current implementation ignores the effect of Document.selectedStylesheetSet
   // and either only collects persistent style, or additionally preferred
   // style when present.
   selected_stylesheet_set_name_ = name;
   MarkDocumentDirty();
 }
 
 void StyleEngine::SetSelectedStylesheetSetName(const String& name) {
   selected_stylesheet_set_name_ = name;
   // TODO(rune@opera.com): Setting Document.selectedStylesheetSet currently
   // has no other effect than the ability to read back the set value using
   // the same api. If it did have an effect, we should have marked the
   // document scope dirty and triggered an update of the active stylesheets
   // from here.
 }
 
 void StyleEngine::SetHttpDefaultStyle(const String& content) {
-  SetPreferredStylesheetSetNameIfNotSet(content);
+  if (!content.IsEmpty())
+    SetPreferredStylesheetSetNameIfNotSet(content);
 }
 
 void StyleEngine::EnsureUAStyleForFullscreen() {
   if (global_rule_set_.HasFullscreenUAStyle())
     return;
   CSSDefaultStyleSheets::Instance().EnsureDefaultStyleSheetForFullscreen();
   global_rule_set_.MarkDirty();
   UpdateActiveStyle();
 }
 
@@ skipping 215 matching lines @@
 }
 
 DEFINE_TRACE_WRAPPERS(StyleEngine) {
   for (auto sheet : injected_author_style_sheets_) {
     visitor->TraceWrappers(sheet);
   }
   visitor->TraceWrappers(document_style_sheet_collection_);
 }
 
 }  // namespace blink