[Fuchsia] Support --enable-logging=stderr and use in browser tests.

- Support --enable-logging and allow it to be used to enable logging
  to stderr.
- Add --enable-logging=stderr to the command-line in WebEngine
  browser-tests.

Bug: 968387
Change-Id: Ie29db5b086beac7867c0b01746006814a9759517
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1732673
Auto-Submit: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683847}