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This dialog explains the implications of accepting remote
assistance and provides a way for the user to decline
assistance.

This ensures that even if the caller (Hangouts) is compromised,
it won't be able to control the user's desktop without user
interaction.

BUG=405139

Review URL: https://codereview.chromium.org/503063004

Cr-Commit-Position: refs/heads/master@{#293200}