Make calls to AtomicString(const String&) explicit in loader/ and fetch/

Make calls to AtomicString(const String&) explicit in load/ and fetch/, and
avoid useless conversions from String to AtomicString when possible.

This is one step towards making the AtomicString(const String&) constructor
explicit. This CL takes care only of the loader/ & fetch/ part so that it is
more easily reviewable.

R=eseidel
BUG=323739

Review URL: https://codereview.chromium.org/123003002

git-svn-id: svn://svn.chromium.org/blink/trunk@164426 bbb929c8-8fbe-4397-9dbb-9b2b20218538