GitLab

Don't require users who have entered the control group to be eligible
for the trial in subsequent launches in order to stay in the control
group. This makes the control group persistent for users who enter
it (just as the experiment group is persistent for users who are
migrated, although the mechanism there is very different) so there
won't be different biasing around crashing or incognito use.

Also fixes a recent regression that would cause force-enabled or
force-disabled users not to report the group.

TBR=rohitrao

BUG=557115
TEST=For a control group user, either terminate in incognito, or
crash. On the next launch, the user should still be in the control
group.

Review URL: https://codereview.chromium.org/1495443006

Cr-Commit-Position: refs/heads/master@{#362992}