Try to fix the "Clobber" button in the buildbot UI.

scripts/slave/recipe_modules/chromium/api.py

Index: scripts/slave/recipe_modules/chromium/api.py
diff --git a/scripts/slave/recipe_modules/chromium/api.py b/scripts/slave/recipe_modules/chromium/api.py
index f7868291e915bfd9a1cdec416e554ec41a8b1702..8302d64369989c100d7fd14c297d3b4f1f21fc5b 100644
--- a/scripts/slave/recipe_modules/chromium/api.py
+++ b/scripts/slave/recipe_modules/chromium/api.py
@@ -490,7 +490,12 @@ class ChromiumApi(recipe_api.RecipeApi):
 
   def clobber_if_needed(self):
     """Add an explicit clobber step if requested."""
-    if self.c.clobber_before_runhooks:
+    # clobber_before_runhooks is true for bots that apply the 'clobber' config,
+    # that is for bots that do clobber bots on every build.
+    # properties.get('clobber') is true on bots that normally don't clobber,
+    # when the "Clobber" button in the buildbot UI is pressed.
+    if (self.c.clobber_before_runhooks or
+        self.m.properties.get('clobber') is not None):
       self.m.file.rmtree('clobber', self.output_dir)
 
   def runhooks(self, **kwargs):