Performance fixes for the "setReturnValue + v8String -> v8setReturnValueString" conversion.

The perf regression ( PerformanceTests/Bindings/undefined-id-getter.html )
didn't happen until a follow-up fix ( https://codereview.chromium.org/19278012
).

Note that we used to have v8String, v8StringOrNull and v8StringOrUndefined
before the conversion started (see https://codereview.chromium.org/18336004/ ).

BUG=261129
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/19594004

git-svn-id: svn://svn.chromium.org/blink/trunk@154648 bbb929c8-8fbe-4397-9dbb-9b2b20218538