ui: Cache the output of View::OnPaint when the View isn't invalid.

ui/compositor/paint_context.h

Index: ui/compositor/paint_context.h
diff --git a/ui/compositor/paint_context.h b/ui/compositor/paint_context.h
index d63f11f7852b44c328b0cef7f855a0c0b1eef8de..f50f4378cd0643a5e34c7450b947bea500cb5788 100644
--- a/ui/compositor/paint_context.h
+++ b/ui/compositor/paint_context.h
@@ -60,6 +60,11 @@ class COMPOSITOR_EXPORT PaintContext {
   // invalid.
   bool CanCheckInvalidated() const { return !invalidation_.IsEmpty(); }
 
+  // When true, if a thing is not invalidated it does not need to paint itself.
+  // When false, everything should provide an output when painting regardless of
+  // being invalidated in order to remain visible.
+  bool EarlyOutOfPaintingWhenNotInvalidated() const { return !!canvas_; }

[sky, 2015/04/23 16:29:00]

Oy! The double negatives make this hard to parse (I'm referring to not invalid,
not !!). EarlyOutOfPaintingWhenValid() is easier to grok.

[danakj, 2015/04/23 16:34:09]

Good call :)

+
   // When true, the |bounds| touches an invalidated area, so should be
   // re-painted. When false, re-painting can be skipped. Bounds should be in
   // the local space with offsets up to the painting root in the PaintContext.
@@ -86,6 +91,9 @@ class COMPOSITOR_EXPORT PaintContext {
   friend class ClipTransformRecorder;
   friend class CompositingRecorder;
   friend class PaintRecorder;
+  // The Cache class also needs to access the DisplayItemList to append its
+  // cache contents.
+  friend class PaintCache;
 
   PaintContext& operator=(const PaintContext& other) = delete;