Mark AtomicString(StringImpl*) constructor as explicit

Mark AtomicString(StringImpl*) constructor as explicit as calling it
has a cost and it is called inadvertently in several places.

R=abarth
BUG=323739

Review URL: https://codereview.chromium.org/92383002

git-svn-id: svn://svn.chromium.org/blink/trunk@162799 bbb929c8-8fbe-4397-9dbb-9b2b20218538