Use the composed tree for the title attributes in Shadow DOM

Source/core/dom/Element.cpp

Index: Source/core/dom/Element.cpp
diff --git a/Source/core/dom/Element.cpp b/Source/core/dom/Element.cpp
index c3848e786e7c5dc45f6e3d87db757a574632ec2a..fab67142222848293ead73cf2a67d91890024378 100644
--- a/Source/core/dom/Element.cpp
+++ b/Source/core/dom/Element.cpp
@@ -72,6 +72,7 @@
 #include "core/dom/Text.h"
 #include "core/dom/custom/CustomElement.h"
 #include "core/dom/custom/CustomElementRegistrationContext.h"
+#include "core/dom/shadow/ComposedTreeTraversal.h"
 #include "core/dom/shadow/InsertionPoint.h"
 #include "core/dom/shadow/ShadowRoot.h"
 #include "core/editing/FrameSelection.h"
@@ -2578,7 +2579,7 @@ AtomicString Element::computeInheritedLanguage() const
             value = toDocument(n)->contentLanguage();
         }
 
-        n = n->parentNode();
+        n = ComposedTreeTraversal::parent(*n);

[hayato, 2015/01/21 09:04:50]

One tough issue around here is that ComposedTreeTraversal::parent() (and similar
functions in ComposedTreeTraversal) requires that distribution is already
calculated. Unless that, it hits ASSERTION.

We must guarantee it by calling Document::updateDistributionIfNeeded().
However, calling it should be *minimized* because it should be considered
non-trivial tasks.

That has been always our headache since it's non-trivial for us to judge whether
distribution is dirty or not at each place where we need ComposedTreeTraversal.

Could you be aware of that and make sure distribution isn't dirty at each place?
I guess it's hard task in general. I'm happy to help that in offline if you
prefer.

[kojii, 2015/01/21 16:46:20]

Understood, thank you for this.

I originally tried |NodeRenderingTraversal::parent()|, which has
|ASSERT(!node.document().childNeedsDistributionRecalc())|. The patch passed
LayoutTests, but browser_tests hit the AF. I guessed it's because browser_tests
does not run full document life cycles, and LayoutTests didn't hit AF means that
the recalc was done.

Could you mind to tell me how I should interpret this result?

     } while (n && value.isNull());
 
     return value;
@@ -2847,8 +2848,10 @@ SpellcheckAttributeState Element::spellcheckAttributeState() const
 
 bool Element::isSpellCheckingEnabled() const
 {
-    for (const Element* element = this; element; element = element->parentOrShadowHostElement()) {
-        switch (element->spellcheckAttributeState()) {
+    for (const Node* node = this; node; node = ComposedTreeTraversal::parent(*node)) {
+        if (!node->isElementNode())
+            continue;
+        switch (toElement(node)->spellcheckAttributeState()) {
         case SpellcheckAttributeTrue:
             return true;
         case SpellcheckAttributeFalse: