Add infrastructure for partial layouts

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 6952785f34e00eac2e3be70f87d1ea008ad848d3..145d5e9d351db9c47760a647bb8be3692155b08f 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -151,6 +151,7 @@
 #include "core/platform/DateComponents.h"
 #include "core/platform/HistogramSupport.h"
 #include "core/platform/Language.h"
+#include "core/platform/ScrollbarTheme.h"
 #include "core/platform/Timer.h"
 #include "core/platform/chromium/TraceEvent.h"
 #include "core/platform/network/HTTPParsers.h"
@@ -1752,6 +1753,9 @@ void Document::updateLayout()
     if (frameView && renderer() && (frameView->layoutPending() || renderer()->needsLayout()))
         frameView->layout();
 
+    if (frameView)
+        frameView->resetPartialLayoutState();
+
     setNeedsFocusedElementCheck();
 }
 
@@ -1798,6 +1802,29 @@ void Document::updateLayoutIgnorePendingStylesheets()
     m_ignorePendingStylesheets = oldIgnore;
 }
 
+void Document::tryPartialUpdateLayoutIgnorePendingStylesheets(RenderObject* stopLayoutAtRenderer)
+{
+    if (stopLayoutAtRenderer) {
+        // Non-overlay scrollbars can cause layout during layout.
+        // FIXME: Temporarily ignore non-overlay scrollbars for test coverage.
+        if (true || ScrollbarTheme::theme()->usesOverlayScrollbars()) {
+            // FIXME: Text autosizing will not work with partial layout.
+            bool canPartialLayout = true;
+            RenderObject* renderer = stopLayoutAtRenderer;
+            do {
+                if (!renderer->supportsPartialLayout()) {
+                    canPartialLayout = false;
+                    break;
+                }
+            } while ((renderer = renderer->parent()));
+            if (canPartialLayout)
+                view()->setStopLayoutAtRenderer(stopLayoutAtRenderer);
+        }
+    }
+
+    updateLayoutIgnorePendingStylesheets();
+}
+
 PassRefPtr<RenderStyle> Document::styleForElementIgnoringPendingStylesheets(Element* element)
 {
     ASSERT_ARG(element, element->document() == this);