Make ExtensionsBrowserClient wrap some utility functions.

Add IsExtensionIncognitoEnabled() and
CanExtensionCrossIncognito() to ExtensionsBrowserClient to
break extensions/browser's dependency on
chrome/browser/extensions/extension_util.h.

BUG=315853

Review URL: https://codereview.chromium.org/152513010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249563 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249563