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It is only used for main frames:
- the initial main frame of a WebContents. https://crrev.com/c/2617521/4
  is a CQ run with an extra DCHECK to demonstrate this.
- main frames that are being resurrected for javascript: URLs.

Bug: 1161170
Change-Id: I49c60cae2c6be54a7b87b991dd77653ddfdc1006
Cq-Do-Not-Cancel-Tryjobs: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617521
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842849}