This patch adds ability to connect multiple sessions to a single context group. This is an experimental feature, which is already supported in test harness.

So far covered runtime domain with tests (and found a bug thanks to the test). More tests to follow in next patches, probably with code adjustments as well.

BUG=chromium:590878

Review-Url: https://codereview.chromium.org/2906153002
Cr-Commit-Position: refs/heads/master@{#45667}