desktop pwas: Remove Webapp.Engagement.EngagementType histogram.

Erase

const char kPwaWindowEngagementTypeHistogram[] =
    "Webapp.Engagement.EngagementType";

and all related browser tests in hosted_app_browsertest.cc

Do not observe SiteEngagement events in HostedAppBrowserController.

Bug: 930927, 918089
Change-Id: I7244b40ac0f6a9597f425057236edd8c159efa87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1506674
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#639395}
4 files changed