Make EmptyLocalFrameClient disallow network requests by default.

As we attempt to limit the uses of blink::Platform’s
CreateDefaultURLLoaderFactory, it makes sense to avoid using it from
EmptyLocalFrameClient - this CL disallows the default
EmptyLocalFrameClient from making network requests.

This CL assumes that WebPagePopupImpl, InspectorOverlayAgent,
ValidationMessageOverlayDelegate should not make any network requests.
After this CL, these classes will continue using EmptyLocalFrameClient
and if they attempt to make fetch a subresource over network, then they
will hit the new NOTREACHED (and probably crash shortly after when
dereferencing the returned, null URLLoaderFactory).

This CL preserves the behavior of SVGImage::SVGImageLocalFrameClient,
DummyPageHolder (test code) and PingLocalFrameClient (test code)
to continue calling blink::Platform's CreateDefaultURLLoaderFactory.

Bug: 891872
Change-Id: I5132fcc66929ff14b25ce2ce636011e1463244f4
Reviewed-on: https://chromium-review.googlesource.com/c/1359051
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614168}
4 files changed