Add the Copresence API.

This CL adds the main Copresence API. This CL is tracking,
https://codereview.chromium.org/438513002.

This CL also requires
https://codereview.chromium.org/419073002 and
https://codereview.chromium.org/433283002 to land to be fully functional.

API tests for this API are being worked on by ckehoe@ and we'll be sending a
follow on CL tracking this one very soon.

Reviews requested,
xiyuan@ - General review.
kalman@ - API review.
isherman@ - histograms.xml OWNERs review.

[email protected], [email protected], [email protected], [email protected]
BUG=365493

Review URL: https://codereview.chromium.org/444513005

Cr-Commit-Position: refs/heads/master@{#288230}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288230 0039d316-1c4b-4281-b951-d872f2087c98
24 files changed