Media Galleries: Run API tests in a temp directory with an injected common.js file. Avoid the need for nested tests to share common JS code.

Review URL: https://codereview.chromium.org/24420003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225917 0039d316-1c4b-4281-b951-d872f2087c98
Move constants from volume_manager.js to util.js.

VolumeManager will be moved to background page. So, to share the constants,
it is necessary to move the values to util.js.

BUG=268817
TEST=Ran browser_tests --gtest_filter="*FileSystemExtensionApiTest*:*FileManagerBrowserTest*:*FileBrowserPrivateApiTest*" and tested manually.

Review URL: https://codereview.chromium.org/24755002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225918 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225918