Fix crash when CreateAccessTokenStore() is not implemented
Do not assume that GetAccessTokenStore() - that internally calls
CreateAccessTokenStore() - will always return a valid object because
it will return NULL in case it is not implemented by the embedder
(like currently in app_shell). Also, do not start location providers
if the AccessTokenStore was not created and return
"permission denied" for location requests.
Review URL: https://codereview.chromium.org/
795093003
Cr-Commit-Position: refs/heads/master@{#312377}