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When unplugging an external display the WindowTreeHost is destroyed, destroying
the window and stopping the capture. However the compositor is destroyed before
the window in the WindowTreeHost destructor.

This change add an extra check that the compositor is valid before dereferencing.

BUG=441747
TEST=on link_freon unplugging external display does not result in crash when
running desktopCapture example against the external display

Review URL: https://codereview.chromium.org/801823002

Cr-Commit-Position: refs/heads/master@{#308168}