V8: Get build environment from MB

scripts/slave/recipes/v8.expected/full_tryserver_v8_v8_win_rel_ng.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 331 matching lines @@
     "cmd": [],
     "name": "trigger",
     "trigger_specs": [
       {
         "builder_name": "v8_win_rel_ng_triggered",
         "properties": {
           "category": "cq",
           "issue": 12853011,
           "master": "tryserver.v8",
           "parent_build_environment": {
-            "GYP_DEFINES": "chromium_win_pch=0 dcheck_always_on=1 fastbuild=1 target_arch=ia32 test_isolation_mode=prepare use_goma=1 v8_target_arch=ia32",
-            "GYP_GENERATORS": "ninja"
+            "GYP_DEFINES": "target_arch=x64 cool_flag=a=1"
           },
           "parent_got_revision": "f2cbea7e89a45f46819777bb78a1f5bd0241f64c",
           "parent_got_revision_cp": "refs/heads/master@{#50110}",
           "parent_got_swarming_client_revision": "e4fee0b18f8163be6097c9c571640038896debc4",
           "patch_project": "v8",
           "patch_storage": "rietveld",
           "patchset": "1",
           "reason": "CQ",
           "requester": "commit-bot@chromium.org",
           "revision": "12345",
           "rietveld": "https://codereview.chromium.org",
           "swarm_hashes": {
             "bot_default": "[dummy hash for bot_default]",
             "ignition": "[dummy hash for ignition]"
           }
         }
       }
     ],
     "~followup_annotations": [
       "@@@STEP_TEXT@v8_win_rel_ng_triggered@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]