[Editing][Serialization] Sanitizing html with @font-face must not crash

The staging document which is used for sanitizing html markup uses a
EmptyLocalFrameClient. Since the client doesn't have a URLLoaderFactory,
the resource loader fails to fetch the font and crashes the renderer.

This CL skips loading remote font resource if the document is for
markup sanitization.

Note that the sanitized markup won't have the resource that is not
fetched.

Bug: 40840595
Change-Id: I122e48b457c5d62841918075775fdb75d89930cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6022000
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Sambamurthy Bandaru <sambamurthy.bandaru@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1421792}