Move VolumeManager from contentWindow to background page.
This CL moves VolumeManager to background page, so that all windows can share
the instance.
Also, this change should reduce the interaction with browser process via private APIs, when the two or more window is opened.
BUG=268817
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.
Review URL: https://codereview.chromium.org/
24957002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226078 0039d316-1c4b-4281-b951-d872f2087c98