[Files.app] Split the JavaScript files into subdirectories: common, background, and foreground

This is cleanup patch. No functionality should be changed.

This patch splits/moves only JavaScript files. The CSS and image files will be changed in the future patches.

BUG=309979
TEST=browser_test passes

R=hirono@chromium.org
TBR=dgozman@chromium.org
# TBRing for the renaming change.

Review URL: https://codereview.chromium.org/39123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231276 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231276