chromeos: Add asynchronous version of GetSystemSalt()

The old synchronous implementation is renamed to GetSystemSaltSync().
The old implementation is marked as deprecated and all users will switch to GetSystemSalt() soon.

BUG=141009
TEST=build
R=satorux@chromium.org
TBR=bbudge@chromium.org for c/b/renderer_host/pepper, benwells@chromium.org for c/b/extensions/api/music_manager_private

Review URL: https://codereview.chromium.org/26697005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227874 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227874