NativeToJSValue return statement(s) instead of expression in code generator 
NativeToJSValue return statement(s) instead of expression in code generator 

This is preparation for support union return type of IDL.
To support that, we need to generate following statements instead of expression.

if (element.member1())
    return toV8Fast(element.member1(), info, collection);
if (element.member2())
    return toV8Fast(element.member2(), info, collection);
ASSERT_NOT_REACHED()

R=haraken@chromium.org
BUG=240176
TEST=No tests. refactoring.

Review URL: https://chromiumcodereview.appspot.com/14910012

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@150502 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs