Disable coverage tests in //testing/buildbot if coverage is not installed.

At the moment we can't assume that the Python `coverage` module is
installed everywhere we might want to run the presubmit tests in
//testing/buildbot. The right fix for this is to add coverage to
the .vpython spec, but in the meantime this disables the check if
it would fail.

R=kbr@chromium.org, jbudorick@chromium.org
BUG=662541

Change-Id: If71bfe7774788776865c8f1f9ecf38aafaddc308
Reviewed-on: https://chromium-review.googlesource.com/809464Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521772}

testing/buildbot/PRESUBMIT.py

@@ -10,7 +10,7 @@ for more details on the presubmit API built into depot_tools.
 
 
 def CommonChecks(input_api, output_api):
-  return input_api.RunTests([
+  commands = [
     input_api.Command(
       name='generate_buildbot_json', cmd=[
         input_api.python_executable, 'generate_buildbot_json.py', '--check'],
@@ -21,16 +21,31 @@ def CommonChecks(input_api, output_api):
         input_api.python_executable, 'generate_buildbot_json_unittest.py'],
       kwargs={}, message=output_api.PresubmitError),
 
-    input_api.Command(
-      name='generate_buildbot_json_coveragetest', cmd=[
-        input_api.python_executable, 'generate_buildbot_json_coveragetest.py'],
-      kwargs={}, message=output_api.PresubmitError),
-
     input_api.Command(
       name='manage', cmd=[
         input_api.python_executable, 'manage.py', '--check'],
       kwargs={}, message=output_api.PresubmitError),
-  ])
+  ]
+  messages = []
+
+  # TODO(crbug.com/662541), TODO(crbug.com/792130): Make this command
+  # run unconditionally once we've added coverage to .vpython and can
+  # safely assume it will be importable.
+  try:
+    import coverage
+    assert coverage  # This silences pylint.
+    commands.append(input_api.Command(
+        name='generate_buildbot_json_coveragetest',
+        cmd=[input_api.python_executable,
+             'generate_buildbot_json_coveragetest.py'],
+        kwargs={}, message=output_api.PresubmitError))
+  except ImportError:
+    messages.append(output_api.PresubmitNotifyResult(
+        'Python\'s coverage module is not installed; '
+        'coverage tests in //testing/buildbot are disabled.'))
+
+  messages.extend(input_api.RunTests(commands))
+  return messages
 
 
 def CheckChangeOnUpload(input_api, output_api):