When rethrowing an exception, print the stack trace of its original site instead of rethrow site.

When rethrowing an exception, print the stack trace of its original site instead of rethrow site.

BUG=60240
TEST=test-api/RethrowStackTrace

[Yang, 2012-02-02 16:02:48]

We now always capture a stack trace when throwing an exception and stash it away
unless we are throwing an exception that we previously already have thrown. 
This way, when throwing it again, we can take the original stack trace that has
been stashed away.

[Vyacheslav Egorov (Chromium), 2012-02-02 20:36:48]

Consider the difference in behavior between:

try {
  something_with_exception();
} catch (e) {
  throw e;
}

and 

try {
  something_with_exception();
} catch (e) {
  try { 
    something_with_exception();
  } catch (a) {
  }
  throw e;
}

Rethrown exceptions would have completely different stacktraces in this case.