[filesapp] Add getContentMetadata to the UITest system

The chrome.mediaGalleries was removed from files app crbug.com/1114622
and a new file manager private API FMP.getContentMetadata was added to
replace it. Add it to the UITest system test FMP impl.

Test: browser_tests --gtest_filter="FileManagerUITest*"
Bug: 1113981
Change-Id: I03f26913f7f0f96f85c74053e458a42bf314e9aa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537453Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827671}

ui/file_manager/file_manager/test/js/chrome_file_manager_private_test_impl.js

@@ -124,6 +124,9 @@ chrome.fileManagerPrivate = {
     // Returns opened|message_sent|failed|empty.
     setTimeout(callback, 0, 'failed');
   },
+  getContentMetadata: (entry, mimeType, includeImages, callback) => {
+    setTimeout(callback, 0);
+  },
   getContentMimeType: (entry, callback) => {
     setTimeout(callback, 0, '');
   },