[Navi] Add missing metrics for remaining pages in onboarding workflow.

Metrics added for:
  - Landing Page
  - Set Default Browser Page
  - Sign In Page
  - Email Interstitial Page

Bug: 874154
Change-Id: I8c9098a343ec8be9dad3b20ec3b7cdc96660d51e
Reviewed-on: https://chromium-review.googlesource.com/c/1371245
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Reviewed-by: Scott Chen <scottchen@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616181}