Reorganize Layout Tree Construction code to be its own function

This patch trivially encapsulates Layout Tree Construction
code in its own function -> Element::buildOwnLayout().

No behaviour change is expected from this patch since it is
merely moving code around.

With lots of help from @bugsnash

Design doc: http://bit.ly/29MBWvf

BUG=595137

Review-Url: https://codereview.chromium.org/2118393002
Cr-Commit-Position: refs/heads/master@{#404584}

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

@@ -1771,13 +1771,10 @@ StyleRecalcChange Element::recalcOwnStyle(StyleRecalcChange change)
     }
 
     if (localChange == Reattach) {
-        AttachContext reattachContext;
-        reattachContext.resolvedStyle = newStyle.get();
-        bool layoutObjectWillChange = needsAttach() || layoutObject();
-        reattach(reattachContext);
-        if (layoutObjectWillChange || layoutObject())
-            return Reattach;
-        return ReattachNoLayoutObject;
+        // TODO(nainar): Remove the style parameter being passed into buildOwnLayout().
+        // ComputedStyle will now be stored on Node and accessed in buildOwnLayout()
+        // using mutableComputedStyle().
+        return buildOwnLayout(*newStyle);
     }
 
     DCHECK(oldStyle);
@@ -1816,6 +1813,17 @@ StyleRecalcChange Element::recalcOwnStyle(StyleRecalcChange change)
     return localChange;
 }
 
+StyleRecalcChange Element::buildOwnLayout(ComputedStyle& newStyle)
+{
+    AttachContext reattachContext;
+    reattachContext.resolvedStyle = &newStyle;
+    bool layoutObjectWillChange = needsAttach() || layoutObject();
+    reattach(reattachContext);
+    if (layoutObjectWillChange || layoutObject())
+        return Reattach;
+    return ReattachNoLayoutObject;
+}
+
 void Element::updateCallbackSelectors(const ComputedStyle* oldStyle, const ComputedStyle* newStyle)
 {
     Vector<String> emptyVector;

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

@@ -650,6 +650,8 @@ private:
     void setInlineStyleFromString(const AtomicString&);
 
     StyleRecalcChange recalcOwnStyle(StyleRecalcChange);
+    // TODO(nainar): Make this const ComputedStyle&.
+    StyleRecalcChange buildOwnLayout(ComputedStyle&);
 
     inline void checkForEmptyStyleChange();