chromium: useVaapi patch fails

[felschr]

Describe the bug
When I set useVaapi = true for chromium I get the following error on rebuild:

applying patch /nix/store/1spb17vn5gjwfr0izvhld1rimn1j8p30-chromium-vaapi.patch
patching file chrome/browser/about_flags.cc
Hunk #1 succeeded at 1738 with fuzz 1 (offset 24 lines).
Hunk #2 FAILED at 2345.
1 out of 2 hunks FAILED -- saving rejects to file chrome/browser/about_flags.cc.rej
patching file chrome/browser/flag_descriptions.cc
Hunk #1 succeeded at 3071 with fuzz 2 (offset -14 lines).
patching file chrome/browser/flag_descriptions.h
Hunk #1 FAILED at 1846.
1 out of 1 hunk FAILED -- saving rejects to file chrome/browser/flag_descriptions.h.rej
patching file gpu/config/software_rendering_list.json
Hunk #1 succeeded at 334 (offset -37 lines).
builder for '/nix/store/4brlz1jaw4ib45f25z3vwgi4f17mp85i-chromium-unwrapped-78.0.3904.97.drv' failed with exit code 1

Expected behavior
Rebuild should succeed.

Metadata

 - system: `"x86_64-linux"`
 - host os: `Linux 5.3.11, NixOS, 20.03pre202088.e89b21504f3 (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.1`
 - channels(root): `"nixos-20.03pre202088.e89b21504f3, home-manager"`
 - channels(felschr): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: chromium

[veprbl]

cc @colemickens @bendlas @ivan

[bendlas]

builds and runs for me, please test on vaapi-enabled machine