[piex-wasm] Add wasm <script> to the FilesApp Image Loader manifest

The piex.js.wasm script resource containing the Emscripten runtime and
the code to load the piex.out.wasm resource, were added to FilesApp in
CL:1597931. Add the piex.js.wasm to the FilesApp Image Loader manifest
to load it in a <script> tag in the Image Loader background page.

Tbr: lucmult@
Bug: 904630, 935285
Change-Id: Ibfa6cb73e28ca2bf9871cd8648389fcb67710e76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1611617Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659463}

ui/file_manager/image_loader/manifest.json

@@ -20,6 +20,7 @@
   "background": {
     "scripts": [
       "chrome://resources/js/assert.js",
+      "piex/piex.js.wasm",
       "background_scripts.js"
     ],
     "persistent": false