Revert of Remove EnableThreadedCompositing from the browser.

content/shell/common/shell_switches.cc

Index: content/shell/common/shell_switches.cc
diff --git a/content/shell/common/shell_switches.cc b/content/shell/common/shell_switches.cc
index a1cce4417066c4b6cdab718628ff4c6c801fc09f..e267cc584ded1f4ea5178ea0c932b43fe885650a 100644
--- a/content/shell/common/shell_switches.cc
+++ b/content/shell/common/shell_switches.cc
@@ -34,9 +34,6 @@
 // whether or not reloading a webpage releases web-related objects correctly.
 const char kEnableLeakDetection[] = "enable-leak-detection";
 
-// Enabled threaded compositing for layout tests.
-const char kEnableThreadedCompositing[] = "enable-threaded-compositing";
-
 // Encode binary layout test results (images, audio) using base64.
 const char kEncodeBinary[] = "encode-binary";