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

scripts/slave/recipe_modules/chromium/api.py

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import re
 
 from recipe_engine import recipe_api
 from recipe_engine import util as recipe_util
 
 class TestLauncherFilterFileInputPlaceholder(recipe_util.InputPlaceholder):
@@ skipping 472 matching lines @@
 
   def ensure_goma(self, canary=False):
     goma_dir = self.m.goma.ensure_goma(canary=canary)
     if goma_dir:
       # TODO(phajdan.jr): goma_dir should always be non-empty.
       self.c.gyp_env.GYP_DEFINES['gomadir'] = goma_dir
       self.c.compile_py.goma_dir = goma_dir
 
   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):
     """Run the build-configuration hooks for chromium."""
 
     # runhooks might write things into the output directory, so clobber before
     # that.
     self.clobber_if_needed()
 
     env = self.get_env()
@@ skipping 261 matching lines @@
       }))
 
   def get_annotate_by_test_name(self, test_name):
     return 'graphing'
 
   def download_lto_plugin(self):
     return self.m.python(
         name='download LTO plugin',
         script=self.m.path['checkout'].join(
             'build', 'download_gold_plugin.py'))