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These are used, e.g., for the Bookmark -> Edit... dialogs on Cocoa and
Views.

This gets WidgetTestInteractive.*Window*ModalWindowDestroyedActivationTest
passing.

WidgetTestInteractive.*System*ModalWindowReleasesCapture is disabled in
this CL. System modal windows are not needed on Mac.

Currently the implementation on MacViews assumes all modal dialogs are
shown using a sheet. On Cocoa, this is not always true for
WebContents-Modal dialogs (e.g. cookies and site data), but it is true
for some WebContents-Modal dialogs (e.g. certificate information).

A follow-up will add logic to ensure WebContents-Modal dialogs are
handled appropriately, and have good test coverage.

After this all tests pass on the current mac trybot configuration with
toolkit_views=1.

BUG=403679

Review URL: https://codereview.chromium.org/993253002

Cr-Commit-Position: refs/heads/master@{#320977}