V8ObjectBuilder::addString support Nullable.

third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h

Index: third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h
diff --git a/third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h b/third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h
index f3bc32a773e9df7d42d8da88065da677d5fa1aef..b5219bcd411ac3abfcec153cd3e4245dfae53a2c 100644
--- a/third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h
+++ b/third_party/WebKit/Source/bindings/core/v8/V8ObjectBuilder.h
@@ -29,7 +29,9 @@ class CORE_EXPORT V8ObjectBuilder final {
   V8ObjectBuilder& addNull(const String& name);
   V8ObjectBuilder& addBoolean(const String& name, bool value);
   V8ObjectBuilder& addNumber(const String& name, double value);
-  V8ObjectBuilder& addString(const String& name, const String& value);
+  V8ObjectBuilder& addString(const String& name,

[Yuki, 2016/11/09 07:25:50]

It's okay to follow the old style, but
    const StringView& name,
    const StringView& value,
is now recommended.

+                             const String& value,
+                             bool isNullable = false);

[Yuki, 2016/11/09 07:25:50]

In general, we should avoid a boolean flag, I think.  Given the following code:
    x.addString("key", value, true);
It's not clear what the last argument means.

Blink's coding style recommended enum values instead.
    x.addString("key", value, V8ObjectBuilder::Nullable);

In this specific case, my preference is:
    x.addStringOrNull("key", value);
then, it's short and crystal clear.

 
   template <typename T>
   V8ObjectBuilder& add(const String& name, const T& value) {