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This CL adds WebContentsModalDialogManager to the sign-in flow web
contents and implements its delegates. This is needed by the webauthn
modal dialog which is in turn needed by the Gaia sign-in flow with
security keys.

Bug: 1167887
Change-Id: Ief107bf16056c5a8ceb3ed7b192b22497957e09c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636399
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844696}