chromium{Beta,Dev}: M84 -> M85 -> M86 (broken)

Mark chromiumDev as broken since the build requires LLVM 11 which is not
yet in Nixpkgs (due to the lack of an RC, see #93324). Build error:
clang (LLVM option parsing): Unknown command line argument '-basic-aa-recphi=0'.  Try: 'clang (LLVM option parsing) --help'
clang (LLVM option parsing): Did you mean '--basicaa-recphi=0'?
ninja: build stopped: subcommand failed.

pkgs/applications/networking/browsers/chromium/default.nix

@@ -44,7 +44,7 @@ let
           sha256 = "00y2d35wvqmx9glaqhfb62wdgbfpwr77v0934nnvh9ks71vnsjqy";
         };
       });
-    } // lib.optionalAttrs (channel == "dev") {
+    } // lib.optionalAttrs (channel == "beta") {
       gnChromium = gn.overrideAttrs (oldAttrs: {
         version = "2020-05-19";
         src = fetchgit {
@@ -53,6 +53,15 @@ let
           sha256 = "0197msabskgfbxvhzq73gc3wlr3n9cr4bzrhy5z5irbvy05lxk17";
         };
       });
+    } // lib.optionalAttrs (channel == "dev") {
+      gnChromium = gn.overrideAttrs (oldAttrs: {
+        version = "2020-07-20";
+        src = fetchgit {
+          url = "https://gn.googlesource.com/gn";
+          rev = "3028c6a426a4aaf6da91c4ebafe716ae370225fe";
+          sha256 = "0h3wf4152zdvrbb0jbj49q6814lfl3rcy5mj8b2pl9s0ahvkbc6q";
+        };
+      });
     });
 
     browser = callPackage ./browser.nix { inherit channel enableWideVine; };