Add a mainfest decription to integration test extensions

Most test extensions manifest's have a descripton: the Files, Gallery,
Video Player, Audio Player test extension do not (how very odd). Let's
give them a description, for consistency with all test extensions.

Bug: 836254
Change-Id: I43dce817da53f9ae7a584f4b30907c3c0649ee85
Reviewed-on: https://chromium-review.googlesource.com/1156118
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579322}

ui/file_manager/integration_tests/audio_player_test_manifest.json

@@ -4,6 +4,7 @@
   "name": "Audio Player test extension",
   "version": "0.1",
   "manifest_version": 2,
+  "description": "Audio Player test extension",
   "web_accessible_resources": ["*"],
   "background": {
     "scripts": [

ui/file_manager/integration_tests/file_manager_test_manifest.json

@@ -5,6 +5,7 @@
   "version": "0.1",
   "incognito": "split",  // Run a separate instance for the incognito mode.
   "manifest_version": 2,
+  "description": "Files test extension",
   "background": {
     "scripts": [
       // List test_util.js and background.js first because the testcase

ui/file_manager/integration_tests/gallery_test_manifest.json

@@ -4,6 +4,7 @@
   "name": "Gallery test extension",
   "version": "0.1",
   "manifest_version": 2,
+  "description": "Gallery test extension",
   "web_accessible_resources": ["*"],
   "background": {
     "scripts": [

ui/file_manager/integration_tests/video_player_test_manifest.json

@@ -4,6 +4,7 @@
   "name": "Video Player test extension",
   "version": "0.1",
   "manifest_version": 2,
+  "description": "Video Player test extension",
   "web_accessible_resources": ["*"],
   "background": {
     "scripts": [