Emoji: Clear canvas between flag tests.

Clear the canvas following the two flag tests following [40582].

Fixes #40616.



git-svn-id: https://develop.svn.wordpress.org/trunk@40609 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/wp-emoji-loader.js

@@ -66,6 +66,9 @@
 				 * To test for support, we try to render it, and compare the rendering to how it would look if
 				 * the browser doesn't render it correctly (white flag emoji + rainbow emoji).
 				 */
+				// Cleanup from previous test.
+				context.clearRect( 0, 0, canvas.width, canvas.height );
+
 				context.fillText( stringFromCharCode( 55356, 57331, 65039, 8205, 55356, 57096 ), 0, 0 );
 				flag = canvas.toDataURL();