[fuzzer] Make parser fuzzer handle pending exceptions.

This ensures exceptions thrown during parsing are properly propagated
into the surrounding {v8::TryCatch} block. Otherwise running more than
one test input in the same Isolate can fail due to pending exceptions. 

R=jochen@chromium.org
BUG=chromium:715037

Change-Id: Iaa5735515dc097d8cb12dcf8672451f3c9503440
Reviewed-on: https://chromium-review.googlesource.com/490047
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45019}

src/isolate.h

@@ -672,7 +672,7 @@ class Isolate {
   // exceptions.  If an exception was thrown and not handled by an external
   // handler the exception is scheduled to be rethrown when we return to running
   // JavaScript code.  If an exception is scheduled true is returned.
-  bool OptionalRescheduleException(bool is_bottom_call);
+  V8_EXPORT_PRIVATE bool OptionalRescheduleException(bool is_bottom_call);
 
   // Push and pop a promise and the current try-catch handler.
   void PushPromise(Handle<JSObject> promise);

test/fuzzer/parser.cc

@@ -36,7 +36,9 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   v8::internal::Handle<v8::internal::Script> script =
       factory->NewScript(source.ToHandleChecked());
   v8::internal::ParseInfo info(script);
-  v8::internal::parsing::ParseProgram(&info, i_isolate);
+  if (!v8::internal::parsing::ParseProgram(&info, i_isolate)) {
+    i_isolate->OptionalRescheduleException(true);
+  }
   isolate->RequestGarbageCollectionForTesting(
       v8::Isolate::kFullGarbageCollection);
   return 0;