Disable WebApkInstallerTest.UpdateSuccess

Has become flaky on kitkat beginning 11-6-2019. See
https://test-results.appspot.com/dashboards/flakiness_dashboard.html#testType=unit_tests&tests=WebApkInstallerTest.UpdateSuccess

TBR: dominickn@chromium.org
Bug: 974729
Change-Id: I0565fc5474046e9afd5e7c79bf946233606831db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662408
Commit-Queue: Matthew Cary (CET) <mattcary@chromium.org>
Reviewed-by: Matthew Cary (CET) <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669635}
1 file changed