ValueSerializer: Promote scheduled exceptions from wasm::ErrorThrower.

wasm::ErrorThrower doesn't actually throw exceptions, it just schedules them.
As a result, this exception isn't handled properly by code which expects
ValueDeserializer to actually throw. For instance, the unit tests use a
TryCatch to catch and handle expected exceptions in unit tests.

Before this patch, I see local unit test failures because a wasm decode test
schedules one, but it isn't caught (and instead causes Context::New to fail
at the beginning of the next test).

BUG=685713

Review-Url: https://codereview.chromium.org/2659483004
Cr-Commit-Position: refs/heads/master@{#42718}

src/value-serializer.cc

@@ -1569,11 +1569,16 @@ MaybeHandle<JSObject> ValueDeserializer::ReadWasmModule() {
   }
 
   // If that fails, recompile.
+  MaybeHandle<JSObject> result;
+  {
     wasm::ErrorThrower thrower(isolate_, "ValueDeserializer::ReadWasmModule");
-  return wasm::CreateModuleObjectFromBytes(
+    result = wasm::CreateModuleObjectFromBytes(
         isolate_, wire_bytes.begin(), wire_bytes.end(), &thrower,
         wasm::ModuleOrigin::kWasmOrigin, Handle<Script>::null(),
         Vector<const byte>::empty());
+  }
+  RETURN_EXCEPTION_IF_SCHEDULED_EXCEPTION(isolate_, JSObject);
+  return result;
 }
 
 MaybeHandle<JSObject> ValueDeserializer::ReadHostObject() {