Copy test data to //extensions

This copies some assets from //chrome test data into //extensions for use with extensions_unittests.

BUG=None
NOTRY=True

Review URL: https://codereview.chromium.org/443473003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287384 0039d316-1c4b-4281-b951-d872f2087c98

extensions/test/data/image_loader/app.json

+{
+  "name": "test",
+  "version": "1",
+  "manifest_version": 2,
+  "icons": { "16": "16.png",
+             "24": "24.png" }
+}