Don't crash if attempting to serialize TYPE_DATA_PIPE bodies.

Actually somehow serializing these is going to be more difficult, but
this will at least help to not crash for tests that hit this codepath.

On of several changes towards making uploading blobs with network service work.

Tested in a separate CL because multiple unrelated bugfixes are needed to make this
all work and testable (https://chromium-review.googlesource.com/c/chromium/src/+/963008).

(see also some of the discussion in https://groups.google.com/a/chromium.org/d/msg/network-service-dev/Jby2gkdDhi8/3GURbFqRAAAJ
where the problem of serializing uploaded blobs in history was mentioned)

Bug: 821878, 777879, 761117
Change-Id: Id60791d26bc2f53c79290305cd25f7a95f247a85
Reviewed-on: https://chromium-review.googlesource.com/963015Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543413}

content/common/page_state_serialization.cc

@@ -706,8 +706,10 @@ void WriteResourceRequestBody(const network::ResourceRequestBody& request_body,
       case network::DataElement::TYPE_BLOB:
         data_element->set_blob_uuid(element.blob_uuid());
         break;
-      case network::DataElement::TYPE_RAW_FILE:
       case network::DataElement::TYPE_DATA_PIPE:
+        NOTIMPLEMENTED();
+        break;
+      case network::DataElement::TYPE_RAW_FILE:
       case network::DataElement::TYPE_CHUNKED_DATA_PIPE:
       case network::DataElement::TYPE_UNKNOWN:
         NOTREACHED();