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The CVDisplayLink is tied to a ScopedTypeRef data member of DisplayLinkMac.
However, there may be other references. Since there's no guarantee that
~DisplayLinkMac() will invoke the last CVDisplayLinkRelease(..) and clear
the callback automatically, clear it explicitly in ~DisplayLinkMac.

BUG=564780

Review URL: https://codereview.chromium.org/1626163002

Cr-Commit-Position: refs/heads/master@{#371648}