[canvaskit] Add API for deleting webgl ctx

Change-Id: I55e70cc1ae6b16d642dd63170e9402bd3c192d5f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343101
Reviewed-by: Nathaniel Nifong <nifong@google.com>
diff --git a/modules/canvaskit/gpu.js b/modules/canvaskit/gpu.js
index 0323596..061822e 100644
--- a/modules/canvaskit/gpu.js
+++ b/modules/canvaskit/gpu.js
@@ -48,7 +48,11 @@
         }
         GL.makeContextCurrent(handle);
         return handle;
-      }
+      };
+
+      CanvasKit.deleteContext = function(handle) {
+        GL.deleteContext(handle);
+      };
 
       // idOrElement can be of types:
       //  - String - in which case it is interpreted as an id of a