misc/wasm: use "self" instead of "window" in web worker contexts

There is no "window" global in a web worker context. Use "self" instead.

Fixes #26192

Change-Id: I6c6f3db6c3d3d9ca00a473f8c18b849bc07a0017
Reviewed-on: https://go-review.googlesource.com/122055
Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Richard Musiol <neelance@gmail.com>

misc/wasm/wasm_exec.js

@@ -27,7 +27,13 @@
 		global.TextEncoder = util.TextEncoder;
 		global.TextDecoder = util.TextDecoder;
 	} else {
-		window.global = window;
+		if (typeof window !== "undefined") {
+			window.global = window;
+		} else if (typeof self !== "undefined") {
+			self.global = self;
+		} else {
+			throw new Error("cannot export Go (neither window nor self is defined)");
+		}
 
 		let outputBuf = "";
 		global.fs = {