gin: fix crash on concat strings passed to ThrowError

V8TypeAsString was passing nullptr to the ConvertFromV8 function, so now
we always pass the isolate. Also adds a DCHECK to ensure that V8ToString
to ConvertFromV8 to ensure it's always passed a non-null Isolate.

Bug: 866767
Change-Id: I715396f309a0c49e4c6bc6ade14b5f0aad8befec
Reviewed-on: https://chromium-review.googlesource.com/1148209Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577597}

gin/arguments.cc

@@ -48,7 +48,7 @@ v8::Local<v8::Context> Arguments::GetHolderCreationContext() {
   return info_->Holder()->CreationContext();
 }
 
-std::string V8TypeAsString(v8::Local<v8::Value> value) {
+std::string V8TypeAsString(v8::Isolate* isolate, v8::Local<v8::Value> value) {
   if (value.IsEmpty())
     return "<empty handle>";
   if (value->IsUndefined())
@@ -56,7 +56,7 @@ std::string V8TypeAsString(v8::Local<v8::Value> value) {
   if (value->IsNull())
     return "null";
   std::string result;
-  if (!ConvertFromV8(NULL, value, &result))
+  if (!ConvertFromV8(isolate, value, &result))
     return std::string();
   return result;
 }
@@ -67,7 +67,7 @@ void Arguments::ThrowError() const {
 
   return ThrowTypeError(base::StringPrintf(
       "Error processing argument at index %d, conversion failure from %s",
-      next_ - 1, V8TypeAsString((*info_)[next_ - 1]).c_str()));
+      next_ - 1, V8TypeAsString(isolate_, (*info_)[next_ - 1]).c_str()));
 }
 
 void Arguments::ThrowTypeError(const std::string& message) const {

gin/converter.h

@@ -255,6 +255,7 @@ GIN_EXPORT v8::Local<v8::String> StringToSymbol(v8::Isolate* isolate,
 template<typename T>
 bool ConvertFromV8(v8::Isolate* isolate, v8::Local<v8::Value> input,
                    T* result) {
+  DCHECK(isolate);
   return Converter<T>::FromV8(isolate, input, result);
 }