Make calls to AtomicString(const String&) explicit in web/

Make calls to AtomicString(const String&) explicit in web/ 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 web/ part so that it is more
easily reviewable.

R=abarth, eseidel
BUG=323739

Review URL: https://codereview.chromium.org/122993002

git-svn-id: svn://svn.chromium.org/blink/trunk@164414 bbb929c8-8fbe-4397-9dbb-9b2b20218538