Avoid using forced layout to trigger paint invalidation for SVG containers

third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h

Index: third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h
diff --git a/third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h b/third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h
index b3e1fee76f54b0702a8245d1e07258392ff4c978..703c1a6e9bb7e8d715868037687148e6248b327d 100644
--- a/third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h
+++ b/third_party/WebKit/Source/core/layout/svg/LayoutSVGContainer.h
@@ -70,8 +70,13 @@ protected:
 
     bool nodeAtFloatPoint(HitTestResult&, const FloatPoint& pointInParent, HitTestAction) override;
 
+    enum class TransformChange {

[pdr., 2016/06/21 00:13:04]

WDYT of adding a small comment here? The enum approach here is the most
maintainable approach I've seen yet but I can see us scratching our heads in the
future about whether checking scale is required.

Trial balloon:
// TransformChange is used to optimize cases where the scale is known to
// static (see: LayoutSVGContainer::layout). Full can be used in the general
// case when the scale change is unknown, or known to change.

[fs, 2016/06/21 08:24:45]

Comment added.

+        None,
+        ScaleInvariant,
+        Full,
+    };
     // Allow LayoutSVGTransformableContainer to hook in at the right time in layout().
-    virtual bool calculateLocalTransform() { return false; }
+    virtual TransformChange calculateLocalTransform() { return TransformChange::None; }
 
     // Allow LayoutSVGViewportContainer to hook in at the right times in layout() and nodeAtFloatPoint().
     virtual void calcViewport() { }