Remove v8::TryCatch from V8StringResource<T>::prepare so that a caller can
catch and ignore the exception.
Add v8::TryCatch to TOSTRING_VOID and TOSTRING_DEFAULT not to change the existing behavior.
Split V8StringResource<T>::prepare to avoid performance regressions caused by
unnecessary v8::TryCatch creation.
This is a refactoring CL and does not change the existing behavior.

BUG=359386

Review URL: https://codereview.chromium.org/242913002

git-svn-id: svn://svn.chromium.org/blink/trunk@172013 bbb929c8-8fbe-4397-9dbb-9b2b20218538