[piexawasm] test extension: use window.Module rather than self.Module

The extenson does not run in other global contexts (web workers, node,
etc) so replace |self| with |window|.

Bug: 935285
Change-Id: Iba1bd8b4057c935ce70351382e130e940e2206e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589078
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#655108}
diff --git a/ui/file_manager/image_loader/piex/extension/background.js b/ui/file_manager/image_loader/piex/extension/background.js
index 83eaafd..5ad04c7 100644
--- a/ui/file_manager/image_loader/piex/extension/background.js
+++ b/ui/file_manager/image_loader/piex/extension/background.js
@@ -14,7 +14,7 @@
     console.log('[piexwasm] failed loading script:', script.src);
   };
 
-  self.Module = {
+  window.Module = {
     onRuntimeInitialized: () => {
       console.log('[piexwasm] runtime loaded');
     },