Migrate //net and //content fuzz targets to use the new FuzzedDataProvider.

The end goal is to delete `base::FuzzedDataProvider` and always use the
version from libFuzzer upstream, which in fact has evolved from the
initial `FuzzedDataProvider` version that was created in Chromium --
kudos to Chrome networking team who, I believe, have implemented that
class originally.

Since this is the first migration CL, I've also slightly updated libFuzzer
docs to make sure new fuzzer authors won't depend on the class living in
//base.

Bug: 907103
Change-Id: I7042664142efce97835e09fc6f1d7c9eff3c5a7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671287
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671709}
43 files changed