Enable the --new-profile-management on about://flags for Mac and Linux

BUG=NONE

Review URL: https://codereview.chromium.org/96903003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237950 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237950
Split work appropriately between device and IO threads.

This is not yet needed but in a pending change, generating device
HMACs will require access to the ResourceContext, which can only be
used on the IO thread.

BUG=none

Review URL: https://codereview.chromium.org/96963002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237951 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237951