Drop "using namespace" in base/allocator

The code style guide does not allow "using namespace" and they
have a tendency to cause problems with jumbo builds. This patch
removes a "using namespace" in base.

Change-Id: I6b97574d6348e1b8df537708e99d89b97d6fc75e
Reviewed-on: https://chromium-review.googlesource.com/c/1443018
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#627356}
1 file changed