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This re-lands #360510 w/ a build fix to  //mojo/shell:apptests
after #360604; It looks like this was broken when we changed
//mojo/shell/application_manager.mojo in #360604 but
for whatever reason //mojo didn't include //mojo/shell:apptests.

With this fix, 'ninja all' should work again in a GN Linux x64 build.

TBR=phajdan.jr@chromium.org, ben@chromium.org, msw@chromium.org
BUG=432959
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_chromium_archive_rel_ng

Review URL: https://codereview.chromium.org/1460013003

Cr-Commit-Position: refs/heads/master@{#361056}