V8: Get build environment from MB

scripts/slave/recipes/v8.expected/full_client_v8_V8_Win32___debug_builder.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]\\scripts\\slave\\kill_processes.py"
     ],
     "name": "taskkill"
   },
   {
@@ skipping 370 matching lines @@
     "name": "package build"
   },
   {
     "cmd": [],
     "name": "trigger",
     "trigger_specs": [
       {
         "builder_name": "V8 Win32 - debug",
         "properties": {
           "parent_build_environment": {
-            "GYP_DEFINES": "chromium_win_pch=0 fastbuild=1 target_arch=ia32 test_isolation_mode=prepare use_goma=1 v8_enable_slow_dchecks=1 v8_optimized_debug=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",
           "revision": "f2cbea7e89a45f46819777bb78a1f5bd0241f64c",
           "swarm_hashes": {
             "bot_default": "[dummy hash for bot_default]",
             "ignition": "[dummy hash for ignition]",
             "mozilla": "[dummy hash for mozilla]",
             "test262": "[dummy hash for test262]"
           }
         }
       }
     ],
     "~followup_annotations": [
       "@@@STEP_TEXT@V8 Win32 - debug@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]