Add the MB config of the official win Clang bots.

tools/mb/mb_config.pyl

Index: tools/mb/mb_config.pyl
diff --git a/tools/mb/mb_config.pyl b/tools/mb/mb_config.pyl
index 9c8e72eccde2f7fab2ca874d5bd53d2a9bbcb61f..142a2372228e90de89e541e0288ecd39c4fd6e39 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -710,11 +710,13 @@
       # but it's not clear if that's actually used anywhere.
       'win': 'gn_official_x86',
       'win-asan': 'gn_official_syzyasan_x86',
+      'win-clang': 'gn_official_clang_x86',
       'win-pgo': [
         'gn_official_chrome_pgo_phase_1_x86',
         'gn_official_chrome_pgo_phase_2_x86',
       ],
       'win64': 'gn_official_x64',
+      'win64-clang': 'gn_official_clang_x64',
       'win64-pgo': [
         'gn_official_chrome_pgo_phase_1_x64',
         'gn_official_chrome_pgo_phase_2_x64',
@@ -1237,6 +1239,14 @@
       'gn', 'official',
     ],
 
+    'gn_official_clang_x64': [
+      'gn', 'clang', 'official', 'x64',
+    ],
+
+    'gn_official_clang_x86': [
+      'gn', 'clang', 'official', 'x86',
+    ],
+
     'gn_official_x86': [
       'gn', 'official', 'x86',
     ],