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Previously, the instance was prioritized when the OS fully started.
However, it turned out that the prioritization caused Chrome UI jank
on some boards.

This CL changes the way of prioritization more dynamic to prevent
such a jank.

BUG=chrome-os-partner:60946
TEST=try

Review-Url: https://codereview.chromium.org/2624513004
Cr-Commit-Position: refs/heads/master@{#442768}