Removing using namespace std as per Blink coding guidelines

As per Blink coding guidelines as mentioned in
http://dev.chromium.org/blink/coding-style
I made the following changes in loader and fetch modules

Review URL: https://codereview.chromium.org/143003030

git-svn-id: svn://svn.chromium.org/blink/trunk@166924 bbb929c8-8fbe-4397-9dbb-9b2b20218538