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This patch makes the Chromium apptest runner resilient to missing apps
(e.g. tracing.mojo) polluting the output of mojo_shell that it tries to
parse.

BUG=450356
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/892933002

Cr-Commit-Position: refs/heads/master@{#314134}