Added a use counter for in-body external stylesheets

This will give us an idea for how common it is to have in-body external
stylesheets and gauge the impact of making them not block paint or
determine if we need stronger protection like blocking the HTML parser.

BUG=481122, 638074

Review-Url: https://codereview.chromium.org/2286793002
Cr-Commit-Position: refs/heads/master@{#415792}

third_party/WebKit/Source/core/core.gypi

@@ -2678,6 +2678,8 @@
             'dom/StyleElement.h',
             'dom/StyleEngine.cpp',
             'dom/StyleEngine.h',
+            'dom/StyleEngineContext.cpp',
+            'dom/StyleEngineContext.h',
             'dom/StyleSheetCandidate.cpp',
             'dom/StyleSheetCandidate.h',
             'dom/StyleSheetCollection.cpp',

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

+/*
+ * Copyright (C) 2016 Google, Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY GOOGLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "core/dom/StyleEngineContext.h"
+
+#include "core/frame/UseCounter.h"
+
+namespace blink {
+
+StyleEngineContext::StyleEngineContext()
+    : m_addedPendingSheetBeforeBody(false)
+{
+}
+
+void StyleEngineContext::addingPendingSheet(const Document& document)
+{
+    // If the sheet was ever added before the body then all references to it are
+    // treated as before-body.
+    if (!m_addedPendingSheetBeforeBody) {
+        m_addedPendingSheetBeforeBody = !document.body();
+        if (!m_addedPendingSheetBeforeBody)
+            UseCounter::count(document, UseCounter::PendingStylesheetAddedAfterBodyStarted);
+    }
+}
+
+} // namespace blink

third_party/WebKit/Source/core/dom/StyleEngineContext.h

@@ -9,12 +9,12 @@
 
 namespace blink {
 
-class StyleEngineContext {
+class CORE_EXPORT StyleEngineContext {
 public:
-    StyleEngineContext() : m_addedPendingSheetBeforeBody(false) {}
+    StyleEngineContext();
     ~StyleEngineContext() {}
     bool addedPendingSheetBeforeBody() const { return m_addedPendingSheetBeforeBody; }
-    void addingPendingSheet(const Document& document) { m_addedPendingSheetBeforeBody = m_addedPendingSheetBeforeBody || !document.body(); }
+    void addingPendingSheet(const Document&);
 private:
     bool m_addedPendingSheetBeforeBody : 1;
 };

third_party/WebKit/Source/core/frame/UseCounter.h

@@ -1275,6 +1275,7 @@ public:
         DraggableAttribute = 1539,
         CleanScriptElementWithNonce = 1540,
         PotentiallyInjectedScriptElementWithNonce = 1541,
+        PendingStylesheetAddedAfterBodyStarted = 1542,
 
         // Add new features immediately above this line. Don't change assigned
         // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/histograms.xml

@@ -80322,6 +80322,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1539" label="DraggableAttribute"/>
   <int value="1540" label="CleanScriptElementWithNonce"/>
   <int value="1541" label="PotentiallyInjectedScriptElementWithNonce"/>
+  <int value="1542" label="PendingStylesheetAddedAfterBodyStarted"/>
 </enum>
 
 <enum name="FetchRequestMode" type="int">