Add swarming support for MB (and the GN bots) in the chromium recipes.

scripts/slave/recipes/chromium.expected/full_chromium_fyi_CrWinClang64_dbg_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]\\scripts\\slave\\kill_processes.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "taskkill"
   },
@@ skipping 69 matching lines @@
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "GYP_CHROMIUM_NO_ACTION": "1"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[SLAVE_BUILD]\\src\\tools\\mb\\mb.py",
-      "gen",
-      "-v",
-      "-m",
-      "chromium.fyi",
-      "-b",
-      "CrWinClang64(dbg)",
-      "--config-file",
-      "[SLAVE_BUILD]\\src\\tools\\mb\\mb_config.pyl",
-      "--goma-dir",
-      "[BUILD]\\goma",
-      "//out/Debug_x64"
-    ],
-    "cwd": "[SLAVE_BUILD]",
-    "name": "generate_build_files"
-  },
-  {
-    "cmd": [
-      "python",
-      "-u",
       "\nimport shutil\nimport sys\nshutil.copy(sys.argv[1], sys.argv[2])\n",
       "[SLAVE_BUILD]\\src\\testing\\buildbot\\chromium.fyi.json",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "read test spec",
     "~followup_annotations": [
       "@@@STEP_TEXT@path: [SLAVE_BUILD]\\src\\testing\\buildbot\\chromium.fyi.json@@@",
       "@@@STEP_LOG_LINE@json.output@{}@@@",
       "@@@STEP_LOG_END@json.output@@@"
@@ skipping 18 matching lines @@
     "name": "get compile targets for scripts",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
+      "[SLAVE_BUILD]\\src\\tools\\mb\\mb.py",
+      "gen",
+      "-v",
+      "-m",
+      "chromium.fyi",
+      "-b",
+      "CrWinClang64(dbg)",
+      "--config-file",
+      "[SLAVE_BUILD]\\src\\tools\\mb\\mb_config.pyl",
+      "--goma-dir",
+      "[BUILD]\\goma",
+      "//out/Debug_x64"
+    ],
+    "cwd": "[SLAVE_BUILD]",
+    "name": "generate_build_files"
+  },
+  {
+    "cmd": [
+      "python",
+      "-u",
       "[BUILD]\\scripts\\slave\\cleanup_temp.py"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "cleanup_temp"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
@@ skipping 44 matching lines @@
       "f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"CrWinClang64(dbg)\", \"buildnumber\": 571, \"mastername\": \"chromium.fyi\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"buildername\": \"CrWinClang64(dbg)\", \"buildnumber\": 571, \"mastername\": \"chromium.fyi\", \"parent_buildername\": null, \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "package build"
   }
 ]