Add unknown file size handling in ComplexFormDataBytesConsumer

third_party/WebKit/LayoutTests/fast/files/blob-reading-from-form-file.html

+<form id="form">
+        <input id="file" multiple type="file" name="file">
+</form>
+
+<script src="../../resources/testharness.js"></script>
+<script src="../../resources/testharnessreport.js"></script>
+<script type="text/javascript">
+
+promise_test((t) => {
+  eventSender.beginDragWithFiles(
+    ['resources/UTF8.txt', 'resources/UTF8-2.txt']);
+  eventSender.mouseMoveTo(10, 10);
+  eventSender.mouseUp();
+
+  var form = document.getElementById('form');
+  var formData = new FormData(form);
+  var request = new Request('/', {
+    method: 'POST',
+    body: formData
+  });
+  return request.text().then(text => {
+    assert_true(text.search('WebKitFormBoundary') > 0,
+                'the boundary is contained');
+    assert_true(text.search('Hello') > 0, 'UTF8.txt is contained');
+    assert_true(text.search('Wonderful') > 0, 'UTF8-2.txt is contained');
+  });
+}, "Reading a text from form files.");
+
+
+promise_test((t) => {
+  eventSender.beginDragWithFiles(['resources/UTF8.txt']);
+  eventSender.mouseMoveTo(10, 10);
+  eventSender.mouseUp();
+
+  var file = document.getElementById('file');
+  var formData = new FormData();
+  formData.append('file', file.files[0]);
+  assert_equals(file.files[0].size, 5, 'file size');
+  var request = new Request('/', {
+    method: 'POST',
+    body: formData
+  });
+  return request.text().then(text => {
+    assert_true(text.search('WebKitFormBoundary') > 0,
+                'the boundary is contained');
+    assert_true(text.search('Hello') > 0, 'UTF8.txt is contained');
+    assert_true(text.search('Wonderful') < 0, 'UTF8-2.txt is not contained');
+  });
+}, "Reading a text from a generated form data.");
+
+</script>
+