Set "application/octet-stream" content type for empty <input type=file>

Empty <input type=file> is represented incorrectly in FormData.
If there are no selected files, then append an entry to entry
list with name and a new File object with an empty name,
application/octet-stream as type, and an empty body [1].

[1] https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#constructing-form-data-set

Bug: 840714
Change-Id: Ifadade1e260e0314664549df2d35f0cd38c76bea
Reviewed-on: https://chromium-review.googlesource.com/1140236Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575931}

third_party/WebKit/LayoutTests/external/wpt/html/semantics/forms/form-submission-0/form-data-set-empty-file.window-expected.txt

-This is a testharness.js-based test.
-FAIL Empty <input type=file> is still serialized assert_equals: type expected "application/octet-stream" but got ""
-Harness: the test ran to completion.
-

third_party/blink/renderer/core/fileapi/file.cc

@@ -26,6 +26,7 @@
 #include "third_party/blink/renderer/core/fileapi/file.h"
 
 #include <memory>
+
 #include "third_party/blink/public/platform/file_path_conversion.h"
 #include "third_party/blink/public/platform/platform.h"
 #include "third_party/blink/renderer/core/fileapi/file_property_bag.h"
@@ -69,6 +70,11 @@ static std::unique_ptr<BlobData> CreateBlobDataForFileWithType(
 static std::unique_ptr<BlobData> CreateBlobDataForFile(
     const String& path,
     File::ContentTypeLookupPolicy policy) {
+  if (path.IsEmpty()) {
+    std::unique_ptr<BlobData> blob_data = BlobData::Create();
+    blob_data->SetContentType("application/octet-stream");
+    return blob_data;
+  }
   return CreateBlobDataForFileWithType(
       path, GetContentTypeFromFileName(path, policy));
 }