This CL fixes an issue with --enable-tracing which was introduced
recently where the tracing file was closed too early. In addition it
adds a test for --enable-tracing to avoid such an issue in the future.

R=machenbach@chromium.org, rmcilroy@chromium.org

Change-Id: I1b3699a4dfbe27230ac1d21d5f6bc7b2ee9ed435
Reviewed-on: https://chromium-review.googlesource.com/796214Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49742}