[Files.appp] Remove element-type check in command execution

Previously, we checked the type of the event target, since some element doesn't support to extract the path. But with CommandUtil.getCommandEntry, all the context-menu elements support it and we don't need to consider it. So we remove it.

BUG=none
TEST=manually tested

Review URL: https://codereview.chromium.org/72993004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237894 0039d316-1c4b-4281-b951-d872f2087c98
Files.app: Remove RootDirectory constant from FileTasks.

Previously FileTasks class used RootDirectory.DOWNLOADS constant to obtain
volume info where the new image is saved.

This CL updates the behavior to obtain the Download directory of current
profile.

BUG=302090
TEST=open the garally, edit and save an image.

Review URL: https://codereview.chromium.org/94433002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237895 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237895