Files.app: Fix flaky transfer test.

Previously the assertion for the files in the transfer test is done as followings:

1. Wait the number of files changed.
2. Then check the properties of files.

So if the check is done after files are added but before the properties are
updated, the test would fail.

The CL fixes the test so that it waits until the expected properties are shown
in the file list.

BUG=347004
TEST=run the test

Review URL: https://codereview.chromium.org/184553002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254454 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254454