Fix accessing SafeBrowsingService::url_request_context_getter_ from multiple threads.

BUG=262999

Review URL: https://codereview.chromium.org/24280010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227697 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227697