Support TYPE_DATA_PIPE in CanReadRequestBody.

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).

Bug: 821878
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I33812ac556029066196b3fcbdb54d6bdbc041a86
Reviewed-on: https://chromium-review.googlesource.com/963007Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543188}

content/browser/child_process_security_policy_impl.cc

@@ -806,6 +806,10 @@ bool ChildProcessSecurityPolicyImpl::CanReadRequestBody(
         // sufficient defense against access from an unrelated renderer.
         break;
 
+      case network::DataElement::TYPE_DATA_PIPE:
+        // Data is self-contained within |body| - no need to check access.
+        break;
+
       case network::DataElement::TYPE_UNKNOWN:
       default:
         // Fail safe - deny access.