Adds icons section in crosh_builtin manifest.

The icon itself lives in //third_party/libapps/nassh/images/dev
and is only used in chromeos. We allow chromeos to scale the icon from 192
to the device-appropriate size.

Bug: 813699
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I01385219919686915abbfddbc7032adff6aab3a0
Reviewed-on: https://chromium-review.googlesource.com/1025470Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553874}

chrome/browser/resources/chromeos/crosh_builtin/manifest.json

 {
   "key": "AAAAB3NzaC1yc2EAAAADAQABAAAAgQDtKfIKWzC4HnQbyAeddk5h78K7LSyrIEnLKDsQCNxhfsavJ+otV9JprDSz3nF7EHZokXiC72SHxdMndt2IlId/aRfhbU4UGidrmFBKg6v1Fs2zey6niB+xLOhJQMe9XtwVNWDpiGXHLmwNhG/FLhj0bBBf1HZDBV18Xp47ymLiuQ==",
-  "name": "Chrome OS developer shell",
+  "name": "Terminal",
   "manifest_version": 2,
+  "icons": {
+    "192": "images/dev/crostini-192.webp"
+  },
   "content_security_policy": "script-src 'self' blob: filesystem:; object-src 'self' blob: filesystem:",
   "version": "0.8.36",
   "default_locale": "en",