Remove "using namespace" statements from headers

It is bad practice to have "using namespace" statements in the headers as it
can change the lookup rules in unrelated implementation files.

This is also against Chromium coding style:
http://www.chromium.org/developers/coding-style/chromium-style-checker-errors#TOC-Using-namespace-in-a-header

R=esprehn, adamk

Review URL: https://codereview.chromium.org/134453006

git-svn-id: svn://svn.chromium.org/blink/trunk@165150 bbb929c8-8fbe-4397-9dbb-9b2b20218538