Switch CrWinClang to gn again.

This relands parts of https://codereview.chromium.org/1947183004/
Now that clobber runs as a separate step before runhooks, swarming should work
on gn clobber bots.

BUG=498033

Review-Url: https://codereview.chromium.org/2043743003
Cr-Commit-Position: refs/heads/master@{#398148}

tools/mb/mb_config.pyl

@@ -118,7 +118,7 @@
       'CrWin7Goma(dbg)': 'gyp_debug_bot_minimal_symbols_x86',
       'CrWin7Goma(dll)': 'gyp_shared_release_bot_minimal_symbols_x86',
       'CrWinClang':
-        'swarming_gyp_clang_official_release_bot_minimal_symbols_x86',
+        'swarming_gn_clang_official_release_bot_minimal_symbols_x86',
       'CrWinClang(dbg)':
         'swarming_gyp_clang_debug_bot_minimal_symbols_x86',
       'CrWinClang(dbg) tester': 'none',
@@ -1736,8 +1736,8 @@
       'minimal_symbols', 'x64',
     ],
 
-    'swarming_gyp_clang_official_release_bot_minimal_symbols_x86': [
-      'swarming', 'gyp', 'clang', 'official', 'release_bot',
+    'swarming_gn_clang_official_release_bot_minimal_symbols_x86': [
+      'swarming', 'gn', 'clang', 'official', 'release_bot',
       'minimal_symbols', 'x86',
     ],