We shouldn't throw under FLAG_debug_code, rather abort.

Throwing under FLAG_debug_code confuses the rest of our infrastructure
which expects a safe point at the site of call into the runtime
for throw. We were doing that to make a clusterfuzz test happy, but
the better solution is to assert/abort under debug_code, and prevent
clusterfuzz from fuzzing on internal APIs that crash on incorrect
values.

We'll need to alter the fuzzer to turn off fuzzing for:

string-natives.js
lithium/SeqStringSetChar.js
regress/regress-seqstrsetchar-ex3.js
regress/regress-seqstrsetchar-ex1.js
regress/regress-crbug-320922.js

So as to prevent the fuzzer from running
%_OneByteSeqStringSetChar() and
%_TwoByteSeqStringSetChar().

BUG=
R=hpayer@chromium.org, machenbach@chromium.org

Review URL: https://codereview.chromium.org/139903005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18878 ce2b1a6d-e550-0410-aec6-3dcde31c8c00