IDL: Improve CG for attributes with [CachedAttribute]

IDL: Improve CG for attributes with [CachedAttribute]

Avoid converting the C++ value to a V8 value twice (once for the call to
V8HiddenValue::setHiddenValue() and once for the function return value)
by storing the converted value in a local and using that in both places.

The similar case attribute.is_keep_alive_for_gc already worked this way,
but with a different variable name ('wrapper' instead of 'v8Value').

The only significant CG change is that the Vector<String> returned by
Navigator::languages() is now converted only once, and that the same
resulting JavaScript array object is returned on the first access as on
subsequent accesses.

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=191980

[Jens Widell, 2015-03-16 14:59:22]

jl@opera.com changed reviewers:
+ haraken@chromium.org

[Jens Widell, 2015-03-16 14:59:22]

PTAL

[haraken, 2015-03-16 23:46:01]

LGTM

[Jens Widell, 2015-03-17 05:56:34]

The CQ bit was checked by jl@opera.com

[commit-bot: I haz the power, 2015-03-17 05:56:47]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1012713003/1

[commit-bot: I haz the power, 2015-03-17 06:00:12]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=191980