cc: Add impl-side painting command line switch.

cc/switches.cc

Index: cc/switches.cc
diff --git a/cc/switches.cc b/cc/switches.cc
index 86409fdd331a0537e7c27ab1d84b93594ad95870..2214c3dba4226654ea42aa8cd8559b41bb976693 100644
--- a/cc/switches.cc
+++ b/cc/switches.cc
@@ -26,5 +26,8 @@ const char kEnablePinchInCompositor[]       = "enable-pinch-in-compositor";
 // content (i.e. jank) instead of showing checkerboards for missing content.
 const char kJankInsteadOfCheckerboard[] = "jank-instead-of-checkerboard";
 
+// Paint content on the compositor thread instead of the main thread.
+const char kImplSidePainting[] = "impl-side-painting";
+
 }  // namespace switches
 }  // namespace cc