[OnionSoup] Introduce blink.mojom.FetchAPIDataElement

This CL introduces a new mojom struct FetchAPIDataElement which has the
same members with network.mojom.DataElement. This struct will be used by
FetchAPIRequestBody.

The typemap of FetchAPIDataElement as below:
  - default: network::DataElement
  - Blink: blink::FormDataElement

BUG=860403

Change-Id: I48de836fccbd4d868618f8742d3e0c8b62081f74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1678955
Commit-Queue: Minggang Wang <[email protected]>
Reviewed-by: Makoto Shimazu <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Cr-Commit-Position: refs/heads/master@{#674763}
diff --git a/third_party/blink/public/mojom/BUILD.gn b/third_party/blink/public/mojom/BUILD.gn
index ff33fe93..f5c8579 100644
--- a/third_party/blink/public/mojom/BUILD.gn
+++ b/third_party/blink/public/mojom/BUILD.gn
@@ -159,6 +159,7 @@
     "//mojo/public/mojom/base",
     "//services/device/public/mojom",
     "//services/network/public/mojom",
+    "//services/network/public/mojom:data_pipe_interfaces",
     "//services/network/public/mojom:websocket_mojom",
     "//services/service_manager/public/mojom",
     "//services/viz/public/interfaces",