Refactor cc scrollbar layers to separate solid-color vs desktop.

cc/base/switches.cc

Index: cc/base/switches.cc
diff --git a/cc/base/switches.cc b/cc/base/switches.cc
index 2dac58b691147a9b073b87274984fad8c5f9099d..1aad816a6b372e6150195293497c1b6903e1b60a 100644
--- a/cc/base/switches.cc
+++ b/cc/base/switches.cc
@@ -131,6 +131,14 @@ const char kUseMapImage[] = "use-map-image";
 // Prevents the layer tree unit tests from timing out.
 const char kCCLayerTreeTestNoTimeout[] = "cc-layer-tree-test-no-timeout";
 
+// Enables overlay scrollbars on Aura or Linux. Does nothing on Mac.
+const char kEnableOverlayScrollbars[] = "enable-overlay-scrollbars";
+
+bool AreOverlayScrollbarsEnabled() {
+  return CommandLine::ForCurrentProcess()->HasSwitch(
+      cc::switches::kEnableOverlayScrollbars);
+}
+
 bool IsImplSidePaintingEnabled() {
   const CommandLine& command_line = *CommandLine::ForCurrentProcess();