bindings: Move UnionTypeConversionMode to V8BindingForCore.h.

We want to remove Nullable.h, but UnionTypeConversionMode needs to remain.
Move it to V8BindingForCore.h, as the enum is generally used in ToImpl()
calls.

While here, document it a little bit.

Bug: 798464
Change-Id: Iecbb481114ad934264dcfb0efffc248bcc420981
Reviewed-on: https://chromium-review.googlesource.com/891019Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#532387}

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

@@ -10,11 +10,6 @@
 
 namespace blink {
 
-enum class UnionTypeConversionMode {
-  kNullable,
-  kNotNullable,
-};
-
 template <typename T>
 class Nullable {
   DISALLOW_NEW();

third_party/WebKit/Source/bindings/core/v8/V8BindingForCore.h

@@ -73,6 +73,16 @@ class LocalDOMWindow;
 class LocalFrame;
 class XPathNSResolver;
 
+// Determines how a V8 -> C++ union conversion should be performed: when the
+// JavaScript value being converted is either undefined or null, kNullable will
+// stop the conversion attempt and the union's IsNull() method will return true.
+// If kNotNullable is used, the other conversion steps listed in
+// https://heycam.github.io/webidl/#es-union will continue being attempted.
+enum class UnionTypeConversionMode {
+  kNullable,
+  kNotNullable,
+};
+
 template <typename CallbackInfo>
 inline void V8SetReturnValue(const CallbackInfo& callback_info,
                              DOMWindow* impl) {