Files.app: Replace currentPath of PreviewPanel with currentEntry.

Current path does not contain file system. To introduce the separate file
system. We should switch to entry.

BUG=323806
TEST=manually

Review URL: https://codereview.chromium.org/90473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237537 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237537
[Files.app] Add a test of default task execution.

This patch adds a test of executing the default task by double click on the file.

BUG=none
TEST=manually tested

Review URL: https://codereview.chromium.org/90563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237538 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237538