Test: Create a fake WebURLLoaderFactory in tests

This patch changes the way we create a WebURLLoaderFactory in tests.
The change is that we will inform the client that the load has resulted
in an error (for both sync and async cases), instead of crashing when
accessing nullptr objects in WebURLLoaderImpl

This ensures that fuzzer tests avoid the async load crash, and can
continue.

R=yhirano@chromium.org, avi@chromium.org

Change-Id: I1c0dd8c37fc6c08c1afb11dbe8b1e63b62b13524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1691145
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677969}
7 files changed