IDL: Conversion to Dictionary is trivial (can't fail)

Conversion from v8::Value to blink::Dictionary simply stores a reference
to the v8::Value in the blink::Dictionary object, so can't throw an
exception.  Adjust v8_conversion_is_trivial() accordingly, to avoid
use of TONATIVE_VOID(_INTERNAL) and thus declaration of unnecessary
v8::TryCatch locals.

BUG=413257

Review URL: https://codereview.chromium.org/587653002

git-svn-id: svn://svn.chromium.org/blink/trunk@182331 bbb929c8-8fbe-4397-9dbb-9b2b20218538