Made junit coverage-dir take directory, not file.

build/android/pylib/junit/test_runner.py

 # Copyright 2014 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 json
 import os
 import tempfile
 
 from devil.utils import cmd_helper
 from pylib import constants
@@ skipping 29 matching lines @@
         jar_args.extend(['-package-filter', self._package_filter])
       if self._runner_filter:
         jar_args.extend(['-runner-filter', self._runner_filter])
       if self._sdk_version:
         jar_args.extend(['-sdk-version', self._sdk_version])
       command.extend(['--jar-args', '"%s"' % ' '.join(jar_args)])
 
       # Add JVM arguments.
       jvm_args = []
       if self._coverage_dir:
-        jvm_args.append('-Demma.coverage.out.file=%s' % self._coverage_dir)
+        if not os.path.exists(self._coverage_dir):
+          os.makedirs(self._coverage_dir)
+        elif not os.path.isdir(self._coverage_dir):
+          raise Exception('--coverage-dir takes a directory, not file path.')
+        jvm_args.append('-Demma.coverage.out.file=%s' % os.path.join(
+            self._coverage_dir, '%s.ec' % self._test_suite))
       if jvm_args:
         command.extend(['--jvm-args', '"%s"' % ' '.join(jvm_args)])
 
       return_code = cmd_helper.RunCmd(command)
       results_list = json_results.ParseResultsFromJson(
           json.loads(json_file.read()))
       return (results_list, return_code)
 
   def TearDown(self):
     pass