tabstrip: Use a PaintRecorder to access the Canvas for painting.

ui/compositor/compositing_recorder.cc

Index: ui/compositor/compositing_recorder.cc
diff --git a/ui/compositor/compositing_recorder.cc b/ui/compositor/compositing_recorder.cc
index 18a8fa2efd6f11a59551cc093c0dd1f7c479e55e..20426af49ce9cfaf0c6bf79e0c08b86557c30de8 100644
--- a/ui/compositor/compositing_recorder.cc
+++ b/ui/compositor/compositing_recorder.cc
@@ -10,13 +10,15 @@
 namespace ui {
 
 CompositingRecorder::CompositingRecorder(const PaintContext& context,
-                                         float opacity)
-    : canvas_(context.canvas()) {
-  canvas_->SaveLayerAlpha(0xff * opacity);
+                                         uint8_t alpha)
+    : canvas_(context.canvas()), saved_(alpha < 255) {
+  if (saved_)
+    canvas_->SaveLayerAlpha(alpha);
 }
 
 CompositingRecorder::~CompositingRecorder() {
-  canvas_->Restore();
+  if (saved_)
+    canvas_->Restore();
 }
 
 }  // namespace ui