[wasm] Enable trap handlers by default in D8 on Linux x64

Bug: v8:5277
Change-Id: I5887e2ac3742350c6cb4e5780e2c1c5d02baa34d
Reviewed-on: https://chromium-review.googlesource.com/673548
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48149}

src/d8.cc

@@ -2774,6 +2774,12 @@ bool Shell::SetOptions(int argc, char* argv[]) {
     }
   }
 
+// On x64 Linux we want to enable the Wasm trap handler by default. Setting
+// the flag here allows the command line argument to still override it.
+#if V8_OS_LINUX && V8_TARGET_ARCH_X64
+  SetFlagsFromString("--wasm-trap-handler");
+#endif
+
   v8::V8::SetFlagsFromCommandLine(&argc, argv, true);
 
   // Set up isolated source groups.

test/mjsunit/wasm/disable-trap-handler.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: --allow-natives-syntax --no-wasm-trap-handler
+
+// This test makes sure that --no-wasm-trap-handler has the correct effect.
+
+assertFalse(%IsWasmTrapHandlerEnabled());