Modifying the code to retrieve a pointer rather than a non-const reference in ash/shelf module.

ResourceBundle::GetSharedInstance() returns a non-const reference, but it will
be changed to return a pointer, so updating the code to use it as a pointer

BUG=289640

Review URL: https://codereview.chromium.org/1119983006

Cr-Commit-Position: refs/heads/master@{#328334}
3 files changed