Actually update FieldTrialEntry's activated field

This changes moves the creation of the field trial entry in shared
memory a earlier, in FinalizeGroupChoice(), and sets the "activated"
field on the entry in NotifyFieldTrialGroupSelection(). We still create
the allocator lazily.

BUG=659212

Review-Url: https://codereview.chromium.org/2449143002
Cr-Commit-Position: refs/heads/master@{#427749}
2 files changed