[piexwasm] Use globalThis instead of assuming it is |window|

The global object could be window, self or global, depending on the JS
environment piex-wasm runs in (Web Page, Web Worker, NodeJS, etc). The
|globalThis| object points to the global object in all cases.

No change in behavior (for files app), no new tests.

Bug: 1132695
Change-Id: I8c6f3d21de29bce63f88352fbe89364d8a76c6bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434077Reviewed-by: Trent Apted <tapted@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811160}

ui/file_manager/image_loader/piex_loader.js

@@ -20,10 +20,10 @@ console.log('[PiexLoader] loaded');
 let PiexWasmModule;
 
 /**
- * |window| var Module defined in page <script src='piex/piex.js.wasm'>.
+ * Module defined by 'piex.js.wasm' script.
  * @type {!PiexWasmModule}
  */
-const PiexModule = /** @type {!PiexWasmModule} */ (window['Module']) || {};
+const PiexModule = /** @type {!PiexWasmModule} */ (globalThis['Module']) || {};
 
 /**
  * Set true if the Module.onAbort() handler is called.
@@ -162,7 +162,7 @@ function readSourceData(source) {
     }
 
     const url = /** @type {string} */ (source);
-    window.webkitResolveLocalFileSystemURL(url, readEntry, failure);
+    globalThis.webkitResolveLocalFileSystemURL(url, readEntry, failure);
   });
 }