Avoid threading races on TraceSamplingThread's members

The original CL was landed in https://codereview.chromium.org/26541005/
but the CL caused a dead lock. This CL fixed the issue.

NOTRY=true

Review URL: https://codereview.chromium.org/28593003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233266 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233266
policy: Remove UserCloudPolicyManagerFactory's dependency on Profile.

Changes all relevant occurrences of Profile by BrowserContext.

BUG=271392
(dubroy@ for policy_ui, tedchoc@ for signin_manager_android.cc, joi@ for DEPS)

R=erg@chromium.org, joaodasilva@chromium.org
TBR=dubroy@chromium.org, joi@chromium.org, tedchoc@chromium.org

Review URL: https://codereview.chromium.org/44083005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233267 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233267