Files.app: Add a test to copy a file between two windows.

BUG=345047
TEST=run the test

Review URL: https://codereview.chromium.org/177183003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253379 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253379