Add documentation for platform/bindings

third_party/WebKit/Source/platform/bindings/V8PerContextData.h

Index: third_party/WebKit/Source/platform/bindings/V8PerContextData.h
diff --git a/third_party/WebKit/Source/platform/bindings/V8PerContextData.h b/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
index b3bf2a737dc63c24d25e4d02826294997edc23d0..e6f2981a90250b884086e722932cf0ec0b90a533 100644
--- a/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
+++ b/third_party/WebKit/Source/platform/bindings/V8PerContextData.h
@@ -58,6 +58,8 @@ enum V8ContextEmbedderDataField {
       gin::kEmbedderBlink),             // NOLINT(readability/enum_casing)
 };
 
+// Used to hold data that is associated with a single v8::Context object, and
+// has a 1:1 relationship with v8::Context.
 class PLATFORM_EXPORT V8PerContextData final {
   USING_FAST_MALLOC(V8PerContextData);
   WTF_MAKE_NONCOPYABLE(V8PerContextData);