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

tools/perf/measurements/record_per_area_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.
 
 from measurements import record_per_area
 from telemetry.core import wpr_modes
-from telemetry.testing import page_test_test_case
+from telemetry.unittest import options_for_unittests
+from telemetry.unittest import page_test_test_case
 from telemetry.unittest import test
-from telemetry.unittest import options_for_unittests
 
 
 class RecordPerAreaUnitTest(page_test_test_case.PageTestTestCase):
   """Smoke test for record_per_area measurement
 
      Runs record_per_area measurement on a simple page and verifies
      that all metrics were added to the results. The test is purely functional,
      i.e. it only checks if the metrics are present and non-zero.
   """
 
   def setUp(self):
     self._options = options_for_unittests.GetCopy()
     self._options.browser_options.wpr_mode = wpr_modes.WPR_OFF
 
   @test.Disabled('android')
   def testRecordPerArea(self):
     ps = self.CreatePageSetFromFileInUnittestDataDir('scrollable_page.html')
     measurement = record_per_area.RecordPerArea()
     results = self.RunMeasurement(measurement, ps, options=self._options)
     self.assertEquals(0, len(results.failures))