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This change adds the following metrics:
- Count of times the help page is opened.
- Count of times the user explicitly navigates from the route details to sink list.
- Count of times the user explicitly navigates from the sink list to route details.
- Count of times the user explicitly navigates from the sink list to cast mode picker.

BUG=547977

Review URL: https://codereview.chromium.org/1548123002

Cr-Commit-Position: refs/heads/master@{#367203}