[inspector] explicitly declare default constructor

Node.js still support older versions of clang, and some of those
versions require us to explicitly declare default constructors for
classes. While updating V8 to 6.7 on Node.js we hit a build failure on
Mac OS X and FreeBSD because there was one constructor not complying
with that rule. This commit fixes it.

R=bmeurer@google.com, franzih@google.com, ofrobots@google.com, yangguo@google.com

Bug: v8:7743
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I1f57f0c88c27e4755c9e05f6fedd9def55d8cb77
Reviewed-on: https://chromium-review.googlesource.com/1050666Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#53132}

src/inspector/wasm-translation.cc

@@ -88,7 +88,13 @@ class WasmTranslation::TranslatorImpl::RawTranslator
   void TranslateBack(TransLocation*) override {}
   const WasmSourceInformation& GetSourceInformation(v8::Isolate*,
                                                     int index) override {
-    static const WasmSourceInformation singleEmptySourceInformation;
+    // NOTE(mmarchini): prior to 3.9, clang won't accept const object
+    // instantiations with non-user-provided default constructors, unless an
+    // empty initializer is explicitly given. Node.js still supports older
+    // clang versions, therefore we must take care when using const objects
+    // with default constructors. For more informations, please refer to CWG
+    // 253 (http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253)
+    static const WasmSourceInformation singleEmptySourceInformation = {};
     return singleEmptySourceInformation;
   }
   const String16 GetHash(v8::Isolate*, int index) override {