Update ScrollAnimator's copy of the layer dimensions after layout.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index 7cfecdc9e3832e5a5a14dc5a2010371b30cbdbd3..c2840a5a9c706fb8c81bed2a5e313f0ee11dfaf8 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -257,6 +257,11 @@ void RenderLayer::updateLayerPositionRecursive()
     if (m_reflectionInfo)
         m_reflectionInfo->reflection()->layout();
 
+    // FIXME(400589): We would like to do this in RenderLayerScrollableArea::updateAfterLayout,
+    // but it depends on the size computed by updateLayerPosition.
+    if (m_scrollableArea)
+        m_scrollableArea->scrollAnimator()->contentsResized();

[ajuma, 2015/02/06 14:41:13]

Calling scrollAnimator() will construct an animator if we don't already have
one. Unless that's really the desired behavior, it'd be better to call
existingScrollAnimator and null-check the result.

+
     // FIXME: We should be able to remove this call because we don't care about
     // any descendant-dependent flags, but code somewhere else is reading these
     // flags and depending on us to update them.