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The cursor surface is parented to the primary root window, which may
change when using multiple displays. This CL fixes a crash caused by
removing the child from the primary root window when the parent is a
different root window or the child is orphaned.

BUG=714832
TEST=No crashes after changing primary display.

Review-Url: https://codereview.chromium.org/2862953002
Cr-Commit-Position: refs/heads/master@{#469543}