Add the MB config for the win_pgo_x64 trybot

This bot has been added in https://chromium-review.googlesource.com/c/502573/

Review-Url: https://codereview.chromium.org/2877853002
Cr-Commit-Position: refs/heads/master@{#471169}

tools/mb/mb_config.pyl

@@ -667,6 +667,10 @@
         '1': 'official_optimize_chrome_pgo_phase_1_x86',
         '2': 'official_optimize_chrome_pgo_phase_2_x86',
       },
+      'win_pgo_x64': {
+        '1': 'official_optimize_chrome_pgo_phase_1',
+        '2': 'official_optimize_chrome_pgo_phase_2',
+      },
       'win_upload_clang': 'release_bot',
       'win_chrome_official': 'official_goma_x86',
     },