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The DesktopEngagementService measures session length times on desktop.
Its name is very similar to the SiteEngagementService, which measures
how much user interaction a site has in total. This CL renames
DesktopEngagement* to DesktopSessionDuration* to clarify its role and
make it more distinct from Site Engagement.

BUG=646216

Review-Url: https://codereview.chromium.org/2333113002
Cr-Commit-Position: refs/heads/master@{#418773}