On iOS the equivalent to content::TestBrowserThreadBundle is web::TestWebThreadBundle. Given the single use case to date I think ifdef'ing the type in a "using" decl is better than a generic solution for now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/89cebb0e044df16603e285bbf3c17788e1800d9e

commit 89cebb0e044df16603e285bbf3c17788e1800d9e
Author: Jun Cai <juncai@chromium.org>
Date: Mon Nov 19 19:12:52 2018

Network Service: Use web::TestWebThreadBundle in the NetworkMetricsProviderTest

This is a follow-up CL from the comments at:
https://chromium-review.googlesource.com/c/chromium/src/+/1274585/11

This CL uses ifdef "using" decl to use web::TestWebThreadBundle on iOS as
the equivalent to content::TestBrowserThreadBundle.

Bug: 898307
Change-Id: I35620aade68dc2b6e9855be49afabd5fcd6dfd5d
Reviewed-on: https://chromium-review.googlesource.com/c/1313450
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Jun Cai <juncai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609382}
[modify] https://crrev.com/89cebb0e044df16603e285bbf3c17788e1800d9e/components/metrics/BUILD.gn
[modify] https://crrev.com/89cebb0e044df16603e285bbf3c17788e1800d9e/components/metrics/net/DEPS
[modify] https://crrev.com/89cebb0e044df16603e285bbf3c17788e1800d9e/components/metrics/net/network_metrics_provider_unittest.cc