[wasm] Remove wrong DCHECK

During instantiation, exceptions can be thrown when looking up the
imports, e.g. because of proxies. If the exception is thrown
internally, before actually calling out to JS code, it won't be
externally caught.
This CL removes the DCHECK that errornously checked that a pending
exception was externally caught.

R=mstarzinger@chromium.org

Bug: chromium:948228
Change-Id: Idbdb340167c1943f78397cc9b310ef5743755726
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547855Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60593}

src/wasm/wasm-engine.cc

@@ -255,7 +255,6 @@ void WasmEngine::AsyncInstantiate(
     // We have to move the exception to the promise chain.
     Handle<Object> exception(isolate->pending_exception(), isolate);
     isolate->clear_pending_exception();
-    DCHECK(*isolate->external_caught_exception_address());
     *isolate->external_caught_exception_address() = false;
     resolver->OnInstantiationFailed(exception);
     thrower.Reset();

test/mjsunit/regress/wasm/regress-948228.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load('test/mjsunit/wasm/wasm-module-builder.js');
+
+var {proxy, revoke} = Proxy.revocable({}, {});
+revoke();
+let builder = new WasmModuleBuilder();
+builder.addImport('m', 'q', kSig_v_v);
+WebAssembly.instantiate(builder.toModule(), proxy);