Removing "using" declarations that import names in the C++ Standard library.(Source/platform/[fonts,text])

This is as per the the coding guidelines for blink.
Do not use "using" declarations of any kind to import names
in the standard template library. Directly qualify the names
at the point they're used instead.

Review URL: https://codereview.chromium.org/516953003

git-svn-id: svn://svn.chromium.org/blink/trunk@181130 bbb929c8-8fbe-4397-9dbb-9b2b20218538