Add the "experimental canvas features" flag, and plumb it through to

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 007bc38b89f7bea02be41fe68dfba6999e5073cc..89295156d915ae87d823cb735e6a60aaae481e6e 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -44,6 +44,10 @@ const char kDisableAccelerated2dCanvas[]    = "disable-accelerated-2d-canvas";
 // Disable antialiasing on 2d canvas.
 const char kDisable2dCanvasAntialiasing[]   = "disable-canvas-aa";
 
+// Enable experimental canvas features, e.g. canvas 2D context attributes
+const char kEnableExperimentalCanvasFeatures[]
+                                        = "enable-experimental-canvas-features";
+
 // Disables accelerated compositing.
 const char kDisableAcceleratedCompositing[] = "disable-accelerated-compositing";