Invalidate the active app-modal dialog last
Invalidate the active app-modal dialog last

Invalidating the active dialog might show another dialog of the
currently closing web contents.

BUG=509078
R=bauerb@chromium.org

Review URL: https://codereview.chromium.org/1230373002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338491}

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