Remove dependency from ExtensionService to app mode utils.

This is done by introducing a function on the generic extensions browser
client to determine if the system is running in forced app mode.

BUG=298537

Review URL: https://codereview.chromium.org/93783002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237742 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237742