Several tests in browser_tests crash when the Tether feature flag is enabled and Tether is invoked via TetherService::Get() from ChromeSessionManager. For instance, see all tests in PushMessagingBrowserTest: https://cs.chromium.org/chromium/src/chrome/browser/push_messaging/push_messaging_browsertest.cc?q=PushMessagingBrowserTest&sq=package:chromium&dr=CSs&l=128

These crashes are due to GcmDriver being shut down too early, and then CryptAuthGCMManagerImpl's destructor segfaults when accessing the GcmDriver (GcmDriver is intended to outlive CryptAuthGCMManager). 

Until all of these tests are corrected, an existing workaround in ChromeSessionManager to avoid invoking Tether during testing will be used. https://chromium-review.googlesource.com/c/585854/ introduces this workaround.