Add [TreatReturnedNullStringAs] support for ScalarValueString

Add [TreatReturnedNullStringAs] support for ScalarValueString

Copy/Paste&Find/Replace of r176370 - same thing for ByteString.

Not strictly needed by any existing IDLs, but trivial code change,
and no code size difference until used.

R=horo@chromium.org,haraken@chromium.org
BUG=379009
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176477

[jsbell, 2014-06-18 20:58:17]

I don't know if this is worth landing, but it won't hurt.

If we do land this, I'll do a follow-up patch which refactors the
webidl-type-mapping.html tests for the common cases of (DOMString, ByteString,
ScalarValueString).

[haraken, 2014-06-18 23:24:46]

LGTM. It's a good thing to make the behavior of DOMString, ByteString and
ScalarValueString consistent.

[jsbell, 2014-06-19 00:01:03]

The CQ bit was checked by jsbell@chromium.org

[commit-bot: I haz the power, 2014-06-19 00:01:50]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jsbell@chromium.org/344693002/1

[horo, 2014-06-19 01:26:58]

lgtm

[commit-bot: I haz the power, 2014-06-19 02:04:23]

Change committed as 176477