Removing "using" delclarations that import names from the STL library.

This is as per the the coding guidelines for blink, which disallows
importing names from the STL library through the "using" declarations.

Review URL: https://codereview.chromium.org/199793011

git-svn-id: svn://svn.chromium.org/blink/trunk@169914 bbb929c8-8fbe-4397-9dbb-9b2b20218538