Skip unnecessary v8::TryCatch in V8StringResource::prepare()

Since we're only interested in detecting and propagating an exception,
check whether v8::Value::ToString()'s return value is an empty handle
instead of using a v8::TryCatch local.

This is faster, since initializing a v8::TryCatch has non-zero cost, and
should also be less binary code (v8::Handle::IsEmpty() is inlined and
simply a null check.)

BUG=413257

Review URL: https://codereview.chromium.org/576323004

git-svn-id: svn://svn.chromium.org/blink/trunk@182476 bbb929c8-8fbe-4397-9dbb-9b2b20218538