[DevTools] Do not report worker console messages twice

When the worker is still alive while DevTools is being opened,
there is a race between console messages coming from page and
worker target creation.

Bug: 908863
Change-Id: I9495d4becada0e4f2bd208c0922b2a2a3ea98af9
Reviewed-on: https://chromium-review.googlesource.com/c/1359039
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614609}
2 files changed