Mark the AtomicString(const String&) constructor as explicit

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=164608

[Inactive, 2014-01-05 21:53:50]

I rebased the patch.

[Inactive, 2014-01-07 20:11:15]

Eric, any objection?

[eseidel, 2014-01-07 20:15:27]

lgtm

[commit-bot: I haz the power, 2014-01-07 20:15:56]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/ch.dumez@samsung.com/123503002/80001

[commit-bot: I haz the power, 2014-01-07 20:23:03]

Change committed as 164608