Stringify RequestInit.body

As specified, RequestInit.body should be stringified, i.e.,
{toString(): () => 'hi!'} should be treated as same as 'hi!'.

Bug: 831076
Change-Id: I4118c0faa9535d62b3db2529bf23716fdc25a997
Reviewed-on: https://chromium-review.googlesource.com/1004561Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550523}

third_party/WebKit/LayoutTests/external/wpt/fetch/api/request/request-init-002.html

@@ -43,7 +43,7 @@
             //not equals: cannot guess formData exact value
             assert_true( bodyAsText.search(expectedTextBody) > -1, "Retrieve and verify request body");
           });
-        }, "Initialize Request's body with " + bodyType);
+        }, `Initialize Request's body with "${body}", ${bodyType}`);
       }
 
       var blob = new Blob(["This is a blob"], {type: "application/octet-binary"});
@@ -56,6 +56,7 @@
       checkRequestInit(blob, "application/octet-binary", "This is a blob");
       checkRequestInit(formaData, "multipart/form-data", "name=\"name\"\r\n\r\nvalue");
       checkRequestInit(usvString, "text/plain;charset=UTF-8", "This is a USVString");
+      checkRequestInit({toString: () => "hi!"}, "text/plain;charset=UTF-8", "hi!");
 
       // Ensure test does not time out in case of missing URLSearchParams support.
       if (window.URLSearchParams) {

third_party/blink/renderer/core/fetch/request_init.cc

@@ -276,11 +276,13 @@ void RequestInit::SetUpBody(ExecutionContext* context,
     content_type_ =
         AtomicString("application/x-www-form-urlencoded;charset=UTF-8");
     body_ = new FormDataBytesConsumer(context, std::move(form_data));
-  } else if (v8_body->IsString()) {
+  } else {
+    String string = NativeValueTraits<IDLUSVString>::NativeValue(
+        isolate, v8_body, exception_state);
+    if (exception_state.HadException())
+      return;
     content_type_ = "text/plain;charset=UTF-8";
-    body_ =
-        new FormDataBytesConsumer(NativeValueTraits<IDLUSVString>::NativeValue(
-            isolate, v8_body, exception_state));
+    body_ = new FormDataBytesConsumer(string);
   }
 }