gnu: ungoogled-chromium: Update to 73.0.3683.86-0.4c7fb6d [security fixes].

The update to Chromium 73 includes more than 60 security fixes.  See the
release announcement for more information:
<https://chromereleases.googleblog.com/2019/03/stable-channel-update-for-desktop_12.html>.

* gnu/packages/chromium.scm (%preserved-third-party-files): Drop
"net/third_party/http2" and "net/third_party/spdy".  Add "v8/src/third_party/siphash".
(%chromium-version): Update to 73.0.3683.86.
(%ungoogled-revision): Update to 4c7fb6d1a86602999f30b58ef8b331b2115c5ad8.
(%debian-revision): New variable.
(ungoogled-chromium-source): Add an origin for Debians packaging scripts.
Adjust for Ungoogled refactoring and add a procedure for applying Debian
patches.  Replace unneeded LET* for LET and reindent.
(ungoogled-chromium)[arguments]: Remove one obsolete substitution, add two others.