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

Slimming paint is now much faster than PicturePile backed recording
for impl-side painting. It's what we're going to ship when we enable
impl-side, so make it the default. Left the disable switch just in case
to try out PicturePile if we need to for some reason until we turn
on impl-side by default.

R=piman@chromium.org, sky
BUG=466426

Review URL: https://codereview.chromium.org/1141583003

Cr-Commit-Position: refs/heads/master@{#329469}

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