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This CL does the following:
- Removes sub headers in the WebUI page ("What this means" and "Why this
  is happening").
- Removes the last section in WebUI (the text under "Why this is
  happening") and replaces the text under "What this means" with the
  final UX strings.
- Reuses the strings in the interstitial and the Web UI.
- Removes the "Learn more" link from the interstitial since it now has
  the same text as the WebUI.

Screenshots:
Interstitial: https://drive.google.com/open?id=1ivy8IypDgqnzGGdgAknQFwvlIYO5myb8
WebUI: https://drive.google.com/open?id=1tVDk0_pK2zcAEanTiNi626OoRIzyQ-iB

Bug: 1014711
Change-Id: I3977c393bd4370f977e14b8a37a0836c8ea97457
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952028Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Auto-Submit: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721866}