Add fileSystemProvider API extension example.

A simple cloud extension example that uses the file system provider API
This example is different from https://codereview.chromium.org/360673004
as it is an extension and not an app.

BUG=415847

R=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/587293002

Cr-Commit-Position: refs/heads/master@{#296579}

chrome/common/extensions/docs/examples/api/fileSystemProvider/archive/manifest.json

 {
-  "name": "Fake Archive Handler",
+  "name": "Fake Archive Handler App",
   "version": "0.1",
   "manifest_version": 2,
-  "description": "Demonstrate File System Provider API usage for archives.",
+  "description": "Demonstrate File System Provider API usage for apps.",
   "permissions": [
     "fileSystemProvider",
     {"fileSystem": ["retainEntries"]},

chrome/common/extensions/docs/examples/api/fileSystemProvider/basic/manifest.json

 {
-  "name": "File System Provider API Example",
+  "name": "File System Provider API Extension Example",
   "version": "0.1",
   "manifest_version": 2,
   "description":
-      "Demonstrate features of the API like mounting, listing directories, etc",
+      "Demonstrate features of the API like mounting, listing directories, etc for extensions.",
   "permissions": [
     "fileSystemProvider"
   ],
-  "app": {
-    "background": {
-      "scripts": [
-        "background.js"
-      ]
-    }
+  "background": {
+    "scripts": [
+      "background.js"
+    ]
   }
 }