Remove ThemeService dependency from ExtensionSystem

This dependency needs to be removed for app_shell to start up with an ExtensionSystem but without Chrome.  It was added in https://codereview.chromium.org/9369013/#ps28044

I see no references to ThemeService in the extensions system startup or shutdown code. Chrome starts and runs normally without this dependency, probably because the ThemeService BCKS is created at profile initialization time. Try jobs with this dependency removed pass.  I suspect whatever code required this dependency in the past has been removed or refactored.

BUG=321714
TEST=browser_tests, unit_tests, running Chrome with and without themes installed and opening/closing incognito and regular windows

Review URL: https://codereview.chromium.org/77853004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236446 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236446
Move std::string instance inside of loop.

This is a followup change to r235191 resulting from a chromium-dev
discussion.

BUG=None

Review URL: https://codereview.chromium.org/78593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236447 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236447