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This adds comments to better explain use cases for adding the special
exceptions to SameSite cookie rules for chrome:// scheme pages that were
added in https://crrev.com/c/1904544.

Bug: 1007320
Change-Id: Iba4f6c20d2a4bbca6297be813ebdc9aa0437dcef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227058
Commit-Queue: Lily Chen <chlily@chromium.org>
Commit-Queue: Maksim Orlovich <morlovich@chromium.org>
Auto-Submit: Lily Chen <chlily@chromium.org>
Reviewed-by: Maksim Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774338}