Made ElementRareData store ComputedStyle on LayoutObject if possible.

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

 /*
  * Copyright (C) 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 David Smith <catfish.man@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 73 matching lines @@
     ElementShadow& ensureShadow()
     {
         if (!m_shadow)
             m_shadow = ElementShadow::create();
         return *m_shadow;
     }
 
     NamedNodeMap* attributeMap() const { return m_attributeMap.get(); }
     void setAttributeMap(NamedNodeMap* attributeMap) { m_attributeMap = attributeMap; }
 
-    ComputedStyle* computedStyle() const { return m_computedStyle.get(); }
-    void setComputedStyle(PassRefPtr<ComputedStyle> computedStyle) { m_computedStyle = computedStyle; }
-    void clearComputedStyle() { m_computedStyle = nullptr; }
+    ComputedStyle* computedStyle() const
+    {
+        DCHECK(!(layoutObject() && m_computedStyle));
+        if (layoutObject())
+            return layoutObject()->mutableStyle();
+        return m_computedStyle.get();
+    }
+    void setComputedStyle(PassRefPtr<ComputedStyle> computedStyle)
+    {
+        if (layoutObject())
+            layoutObject()->setStyleInternal(computedStyle);

[esprehn, 2016/09/24 02:10:52]

This case isn't reachable (outside your test which doesn't look right), you
can't end up inside ElementRareData::setComputedStyle when there's a
layoutObject. That code is only reached from Element::ensureComputedStyle

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/dom/Eleme...

when mutableComputedStyle was null, which can only happen if layoutObject() was
null, since layout objects always have a style except very briefly inside
LayoutTreeBuilder between the create call and the setStyle call (we could probs
fix that by passing the style into the constructor, WebKit did it a while ago,
then LayoutObject::style could return a reference).

Note also that using setStyleInternal here is bad since it potentially stuffs an
invalid style into a layout object. This patch worked because the code was
unreachable. setStyleInternal exists for dealing with some complex style sharing
situations. It's also gotten used in a few other cases, but they're super
subtle. Generally speaking you should never use setStyleInternal unless the
newStyle you're setting would return true from ComputedStyle::operator== (which
is expensive, so you don't want to check that yourself outside the recalc
phase).

I know there's a few other callers, like the TextAutosizer. Its usage of
setStyleInternal is technically correct, even though it violates the operator==
requirement, because of how it works inside layout updating styles recursively
down the tree, but I'll admit it's not obvious.

+        else
+            m_computedStyle = computedStyle;
+    }
+    void clearComputedStyleIfNoLayoutObject()
+    {
+        DCHECK(!(layoutObject() && m_computedStyle));

[esprehn, 2016/09/24 02:10:52]

Plz run demorgans :D running it in my head is hard, this check is:

DCHECK(!layoutObject() || !m_computedStyle)

so we can end up in here when there's a layoutObject as long as m_computedStyle
was already null. That's apparent inside Element::recalcStyle where we call this
totally ignoring the state of if there's a layout object.

I'd remove the long IfNoLayoutObject thing here and just leave this as
clearComputedStyle(). This method shouldn't need to care about if there's a
layout object.

+        m_computedStyle = nullptr;
+    }
 
     ClassList* classList() const { return m_classList.get(); }
     void setClassList(ClassList* classList) { m_classList = classList; }
     void clearClassListValueForQuirksMode()
     {
         if (!m_classList)
             return;
         m_classList->clearValueForQuirksMode();
     }
 
@@ skipping 140 matching lines @@
 inline CompositorProxiedPropertySet& ElementRareData::ensureCompositorProxiedPropertySet()
 {
     if (!m_proxiedProperties)
         m_proxiedProperties = CompositorProxiedPropertySet::create();
     return *m_proxiedProperties;
 }
 
 } // namespace blink
 
 #endif // ElementRareData_h