Suppress failures from taskkill when killing content_shell on Windows.
Suppress failures from taskkill when killing content_shell on Windows.

BUG=https://github.com/dart-lang/sdk/issues/29599
R=terry@google.com

Review-Url: https://codereview.chromium.org/2875043002 .

dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk