add win7_chromium_rel_loc_exp to mb_config.pyl

To fix
https://luci-milo.appspot.com/buildbot/tryserver.chromium.win/win7_chromium_rel_loc_exp/266

Bug: 779440
Change-Id: I234789b93011b9b959dfe71fda10e4ad1ab04dac
Reviewed-on: https://chromium-review.googlesource.com/757839Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Cr-Commit-Position: refs/heads/master@{#515530}

tools/mb/mb_config.pyl

@@ -711,6 +711,7 @@
 
     'tryserver.chromium.win': {
       'win7_chromium_rel_ng': 'gpu_tests_release_trybot_x86_minimal_symbols',
+      'win7_chromium_rel_loc_exp': 'gpu_tests_release_trybot_x86_minimal_symbols',
       'win10_chromium_x64_rel_ng': 'release_trybot',
       'win10_chromium_x64_rel_ng_exp': 'release_trybot',
       'win_chromium_gn_upload': 'release_bot_x86_minimal_symbols',