This enables linter checking for "readability/namespace" violations
during presubmit and instead marks the few known exceptions that we
allow explicitly.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1371083003

Cr-Commit-Position: refs/heads/master@{#31019}

BUG=chromium:490559
LOG=NO

Review URL: https://codereview.chromium.org/1352453004

Cr-Commit-Position: refs/heads/master@{#30944}