Generate simple code for "trivial" conversions

A trivial conversion is defined as one that cannot throw an exception,
and thus need no v8::TryCatch or ExceptionState, and no "did it succeed?"
check after the conversion.

For such conversions, generate the code

  Type variable = convertedValue()

instead of

  TONATIVE_VOID(Type, variable, convertedValue())

Also include all such conversions in the set of argument conversions that
don't require a v8::TryCatch, since they then no longer do.

BUG=413257

Review URL: https://codereview.chromium.org/564063002

git-svn-id: svn://svn.chromium.org/blink/trunk@181897 bbb929c8-8fbe-4397-9dbb-9b2b20218538