GitLab

Rename MergeSession in SigninTracker.

Further cleanup as part of the MergeSession/GCMS rework. https://docs.google.com/document/d/1FfmSS7M87L_2tkVuHO3NuruborFm6qHXe2VdktZg-HQ

BUG=466799, 471210

Review URL: https://codereview.chromium.org/1104153002

Cr-Commit-Position: refs/heads/master@{#327268}