ui: Turn on slimming paint by default with impl-side painting.

ui/compositor/compositor_switches.cc

Index: ui/compositor/compositor_switches.cc
diff --git a/ui/compositor/compositor_switches.cc b/ui/compositor/compositor_switches.cc
index 7b8fed3ca9890f634a8b89575913a91cd21a18ab..a8cb8017515b050c25e0c07cde0ee3af5f1b398f 100644
--- a/ui/compositor/compositor_switches.cc
+++ b/ui/compositor/compositor_switches.cc
@@ -22,7 +22,7 @@ const char kUIEnableCompositorAnimationTimelines[] =
 
 const char kUIEnableImplSidePainting[] = "ui-enable-impl-side-painting";
 
-const char kUIEnableSlimmingPaint[] = "ui-enable-slimming-paint";
+const char kUIDisableSlimmingPaint[] = "ui-disable-slimming-paint";
 
 const char kUIEnableZeroCopy[] = "ui-enable-zero-copy";
 
@@ -52,7 +52,7 @@ bool IsUIOneCopyEnabled() {
 bool IsUISlimmingPaintEnabled() {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
-  return command_line.HasSwitch(switches::kUIEnableSlimmingPaint);
+  return !command_line.HasSwitch(switches::kUIDisableSlimmingPaint);
 }
 
 }  // namespace ui