Files.app: Stop to obtaining the background multiple times.

Currently background page is obtained via chrome.runtime.getBackgroundPage
everywhere but it takes about 50ms.

This CL let the application window obtain the background page only once and pass
it the components that require the background page.

BUG=307822
TEST=manually

Review URL: https://codereview.chromium.org/27467002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228915 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228915