Storage of ComputedStyle separate from LayoutObject.

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

Index: third_party/WebKit/Source/core/dom/NodeComputedStyle.h
diff --git a/third_party/WebKit/Source/core/dom/NodeComputedStyle.h b/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
index 78872cdcf125b3139e08a8e7752e6eda40cfea34..a5a9f39ea3c6d12e0662013b8a89dd45807f95d0 100644
--- a/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
+++ b/third_party/WebKit/Source/core/dom/NodeComputedStyle.h
@@ -25,8 +25,10 @@
 #ifndef NodeComputedStyle_h
 #define NodeComputedStyle_h
 
+#include "core/dom/ElementRareData.h"
 #include "core/dom/LayoutTreeBuilderTraversal.h"
 #include "core/dom/Node.h"
+#include "core/dom/NodeRareData.h"
 #include "core/dom/shadow/InsertionPoint.h"
 #include "core/html/HTMLOptGroupElement.h"
 #include "core/layout/LayoutObject.h"
@@ -41,14 +43,19 @@ inline const ComputedStyle* Node::computedStyle() const
 
 inline ComputedStyle* Node::mutableComputedStyle() const
 {
-    if (LayoutObject* layoutObject = this->layoutObject())
-        return layoutObject->mutableStyle();
+    if (hasLayoutObject())
+        return layoutObject()->mutableStyle();
     // <option> and <optgroup> can be styled even if they don't get layout objects,
     // so they store their style internally and return it through nonLayoutObjectComputedStyle().
     // We check here explicitly to avoid the virtual call in the common case.
     if (isHTMLOptGroupElement(*this) || isHTMLOptionElement(this))
         return nonLayoutObjectComputedStyle();
-    return 0;
+    if (hasRareData()) {

[Bugs Nash, 2016/07/19 01:20:36]

Alternatively I could land the patch without this section, just leave it as
'return 0' for now (no change to functionality). This definitely doesn't cause
the bots to crash. This section would however need to be added before any
further progress can be made, as there's no point storing ComputedStyle on
Node's m_data union if it an't be accessed that way.

[esprehn, 2016/07/19 01:59:10]

Crashing sounds like bug... Why does it crash?

+        NodeRareData* rareData = this->rareData();
+        DCHECK(rareData->isElementRareData());

[esprehn, 2016/07/19 03:25:02]

this check is not right, ex.

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/css/resol...

callers this on a Document, if you were correctly setting the hasLayoutObject
flag in this patch you'd hit the branch above since a Document should always
have a layout object, but you don't, so this would crash if the document had
rare data.

[esprehn, 2016/08/05 21:26:26]

Yeah I see how the flag is set now, I'm still nervous about this assert, calling
mutableComputedStyle() on a detached document or a Text node with RareData
should not crash.

What makes you think we never come in here for a Text node?

[Bugs Nash, 2016/08/12 03:55:33]

Patch 9 of this CL moved the ComputedStyle from ElementRareData to NodeRareData
like you suggested. Tim suggested moving it back because Element seemed to be
the only type of Node that has a ComputedStyle.

Tim, any more info on that? 

+        return static_cast<ElementRareData*>(rareData)->computedStyle();
+    }
+    return m_data.m_computedStyle;
 }
 
 inline const ComputedStyle* Node::parentComputedStyle() const