[asm.js] Gracefully handle stack overflow in start function.

R=clemensh@chromium.org
TEST=mjsunit/regress/regress-crbug-762472
BUG=chromium:762472

Change-Id: I8977fb9c9330f03641291b08cf803d0117b7a96a
Reviewed-on: https://chromium-review.googlesource.com/652478Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47852}

src/asmjs/asm-js.cc

@@ -377,7 +377,9 @@ MaybeHandle<Object> AsmJs::InstantiateAsmWasm(Isolate* isolate,
   MaybeHandle<Object> maybe_module_object =
       wasm::SyncInstantiate(isolate, &thrower, module, foreign, memory);
   if (maybe_module_object.is_null()) {
-    DCHECK(!isolate->has_pending_exception());
+    // An exception caused by the module start function will be set as pending
+    // and bypass the {ErrorThrower}, this happens in case of a stack overflow.
+    if (isolate->has_pending_exception()) isolate->clear_pending_exception();
     thrower.Reset();  // Ensure exceptions do not propagate.
     ReportInstantiationFailure(script, position, "Internal wasm failure");
     return MaybeHandle<Object>();

test/mjsunit/regress/regress-crbug-762472.js

+// Copyright 2017 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.
+
+// Flags: --stack-size=100
+
+function Module() {
+  "use asm";
+  function f() {}
+  return { f:f }
+}
+
+function InstantiateNearStackLimit() {
+  try {
+    var fuse = InstantiateNearStackLimit();
+    if (fuse == 0) Module();
+    return fuse - 1;
+  } catch(e) {
+    return init_fuse;
+  }
+}
+
+var init_fuse = 0;
+for (init_fuse = 0; init_fuse < 10; init_fuse++) {
+  InstantiateNearStackLimit();
+}