Reland: Generate separate files for union type containers

third_party/WebKit/Source/bindings/IDLUnionTypes.md

Index: third_party/WebKit/Source/bindings/IDLUnionTypes.md
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+# How to use Blink IDL Union Types
+
+Using [IDL union types](https://heycam.github.io/webidl/#idl-union) in Blink is a bit tricky. Here are some tips to use union types correctly.
+
+## Generated classes
+
+For each union type, the code generator creates a C++ class which is used like an "impl" class of a normal interface type. The name of a generated class is a [type name](https://heycam.github.io/webidl/#dfn-type-name) of the union type. For example, the code generator will create `StringOrFloat` class for `(DOMString or float)`.
+
+The include paths for generated classes depend on the places union types are used. If a union type is used only by IDL files under modules/, the include path is `bindings/modules/v8/FooOrBar.h`. Otherwise, the include path is `bindings/core/v8/FooOrBar.h`. For example, given following definitions:
+
+```webidl
+// core/fileapi/FileReader.idl
+readonly attribute (DOMString or ArrayBuffer)? result;
+
+// dom/CommonDefinitions.idl
+typedef (ArrayBuffer or ArrayBufferView) BufferSource;
+
+// modules/encoding/TextDecoder.idl
+DOMString decode(optional BufferSource input, optional TextDecodeOptions options);
+
+// modules/fetch/Request.idl
+typedef (Request or USVString) RequestInfo;
+```
+
+The include paths will be:
+* bindings/core/v8/StringOrArrayBuffer.h
+* bindings/core/v8/ArrayBufferOrArrayBufferView.h
+* bindings/modules/v8/RequestOrUSVString.h
+
+Note that `ArrayBufferOrArrayBufferView` is located under core/ even it is used by `Request.idl` which is located under modules/.
+
+**Special NOTE**: If you are going to use a union type under core/ and the union type is currently used only under modules/, you will need to update the include path for the union type under modules/.
+
+## Updating GN/GYP files
+TODO(bashi): Mitigate the pain of updating GN/GYP files.
+
+Due to the requirements of GN/GYP, we need to put generated file names in gni/gypi files. Please update `bindings/core/v8/generated.{gni,gypi}` and/or `bindings/modules/v8/generated.{gni,gypi}` accordingly.