Add a telemetry measurement for property tree performance.

cc/base/switches.cc

Index: cc/base/switches.cc
diff --git a/cc/base/switches.cc b/cc/base/switches.cc
index d34821e0fac6bf348af5fd66c7d282d33b484a1e..ffef66cf7e16662ac0313fd2a5d11d1f26ac35ca 100644
--- a/cc/base/switches.cc
+++ b/cc/base/switches.cc
@@ -58,6 +58,11 @@ const char kStrictLayerPropertyChangeChecking[] =
 const char kEnablePinchVirtualViewport[] = "enable-pinch-virtual-viewport";
 const char kDisablePinchVirtualViewport[] = "disable-pinch-virtual-viewport";
 
+// Ensures that the draw properties computed via the property trees math those
+// computed by CalcDrawProperties.
+const char kEnablePropertyTreeVerification[] =
+    "enable-property-tree-verification";
+
 // Disable partial swap which is needed for some OpenGL drivers / emulators.
 const char kUIDisablePartialSwap[] = "ui-disable-partial-swap";