Remove unsafe cast from ImageTransportSurfaceOverlayMac::ScheduleCALayer.

ui/gl/gl_image.h

Index: ui/gl/gl_image.h
diff --git a/ui/gl/gl_image.h b/ui/gl/gl_image.h
index 929970ea9e43b35546cf62284012e1640f65b3a5..8a7a21203f9404196d8a10caae8afc27ca042ae5 100644
--- a/ui/gl/gl_image.h
+++ b/ui/gl/gl_image.h
@@ -85,6 +85,13 @@ class GL_EXPORT GLImage : public base::RefCounted<GLImage> {
   // removed. https://crbug.com/581777#c36
   virtual bool EmulatingRGB() const;
 
+  // An identifier for subclasses. Necessary for safe downcasting.
+  enum class Type {
+    NONE,
+    IOSURFACE
+  };
+  virtual Type GetType() const;
+
  protected:
   virtual ~GLImage() {}