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This CL also splits the IncognitoObserver class into separate
implementations for Android and desktop.

Bug: 268984
Change-Id: Ic59988fe8d08a5217400dffd78eff3d7838f7630
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1715533Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680503}