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Part 1 of several. This is currently set up to be landed as a standalone part of the first run flow work.

This change includes:
- Add strings for Chromium specific first run flow.
- Add first run flow nux to the WebUI (but stay unstamped for now). Used redlines and mocks as guidelines.

BUG=560457

Review URL: https://codereview.chromium.org/1554663002

Cr-Commit-Position: refs/heads/master@{#367221}