Remove "using namespace" from content_shell code

"using namespace" is not allowed per the Code Style Guide, but
worse, much much much worse, it won't work in jumbo builds because
they will be in a different context where a compiler error
will be triggered.

Bug: 886902
Change-Id: I635ceb8ec5d65a9d599ecd38bf0c150c90edf056
Reviewed-on: https://chromium-review.googlesource.com/1233837
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#592717}
4 files changed