add support for forcing compositing mode

chrome/common/chrome_switches.cc

Index: chrome/common/chrome_switches.cc
diff --git a/chrome/common/chrome_switches.cc b/chrome/common/chrome_switches.cc
index 01e4d6a11884f95864df80ff5e192a360be4e358..7286e94785b7be519194ab5cd7483721e1f5ef27 100644
--- a/chrome/common/chrome_switches.cc
+++ b/chrome/common/chrome_switches.cc
@@ -611,6 +611,10 @@ const char kFirstRun[]                      = "first-run";
 // timed out, etc. Useful for testing.
 const char kForceAppsPromoVisible[]         = "force-apps-promo-visible";
 
+// If accelerated compositing is supported, always enter compositing mode for
+// the base layer even when compositing is not strictly required.
+const char kForceCompositingMode[]          = "force-compositing-mode";
+
 // Some field tests may rendomized in the browser, and the randomly selected
 // outcome needs to be propogated to the renderer.  For instance, this is used
 // to modify histograms recorded in the renderer, or to get the renderer to