[jumbo] Resolve symbol clashes in content/shell

Rename two TestServiceImpl to not clash in jumbo build experiments,
and change a kIllegalString constant to be shared.

Bug: 886902
Change-Id: I513e0641834476bb198cf24d71ef088634458313
Reviewed-on: https://chromium-review.googlesource.com/1233709
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#593125}
5 files changed