[telemetry] Create BrowserTestCase to reuse the browser for browser_unittest.

tools/perf/measurements/screenshot_unittest.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 logging
 import shutil
 import tempfile
 
 from measurements import screenshot
 from telemetry import benchmark
 from telemetry.page import page_test
-from telemetry.testing import page_test_test_case
 from telemetry.unittest import options_for_unittests
+from telemetry.unittest import page_test_test_case
 
 
 class ScreenshotUnitTest(page_test_test_case.PageTestTestCase):
   def setUp(self):
     self._options = options_for_unittests.GetCopy()
     self._options.png_outdir = tempfile.mkdtemp('_png_test')
 
   def tearDown(self):
     shutil.rmtree(self._options.png_outdir)
 
   @benchmark.Disabled('win')  # http://crbug.com/386572
   def testScreenshot(self):
     ps = self.CreatePageSetFromFileInUnittestDataDir('blank.html')
     measurement = screenshot.Screenshot()
     try:
       results = self.RunMeasurement(measurement, ps, options=self._options)
     except page_test.TestNotSupportedOnPlatformFailure as failure:
       logging.warning(str(failure))
       return
 
     saved_picture_count = results.FindAllPageSpecificValuesNamed(
         'saved_picture_count')
     self.assertEquals(len(saved_picture_count), 1)
     self.assertGreater(saved_picture_count[0].GetRepresentativeNumber(), 0)