Disable flaky tests in ExtensionManagementApiTest

Two flaky tests were found on Mac10.12:
ExtensionManagementApiTest.LaunchTabApp
ExtensionManagementApiTest.LaunchType

TBR=tbarzic@chromium.org
NOTRY=true

Bug: 915399
Change-Id: If624729d8abb0a68b675d247537de9b0f1cc355e
Reviewed-on: https://chromium-review.googlesource.com/c/1378852
Commit-Queue: Makoto Shimazu <shimazu@chromium.org>
Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617036}
1 file changed