mb: Add a recipe for the chromium.perf.fyi Win Clang Builder

BUG=649539

Review-Url: https://codereview.chromium.org/2358083004
Cr-Commit-Position: refs/heads/master@{#420569}

tools/mb/mb_config.pyl

@@ -294,6 +294,7 @@
 
     'chromium.perf.fyi': {
       'Win Builder': 'official_goma',
+      'Win Clang Builder': 'official_clang',
     },
 
     'client.nacl.sdk': {