Mark the AtomicString(const String&) constructor as explicit

Mark the AtomicString(const String&) constructor as explicit to avoid implicit
conversion from String to StringImpl. Such conversion can be expensive when the
String's internal StringImpl is not already atomic. Therefore, I believe it is
a good idea to make this conversion explicit and avoid it when possible (i.e.
construct directly an AtomicString if we know we are going to need an
AtomicString).

BUG=323739
R=eseidel

Review URL: https://codereview.chromium.org/123503002

git-svn-id: svn://svn.chromium.org/blink/trunk@164608 bbb929c8-8fbe-4397-9dbb-9b2b20218538