Make telemetry platform a singleton

tools/telemetry/telemetry/core/platform/platform_backend_unittest.py

Index: tools/telemetry/telemetry/core/platform/platform_backend_unittest.py
diff --git a/tools/telemetry/telemetry/core/platform/platform_backend_unittest.py b/tools/telemetry/telemetry/core/platform/platform_backend_unittest.py
index d4267f7fa6c5ea30d39cf72b962c18bb3da4f1e0..6db32419f40e6e3373f74c3a47577471c3f1f9b3 100644
--- a/tools/telemetry/telemetry/core/platform/platform_backend_unittest.py
+++ b/tools/telemetry/telemetry/core/platform/platform_backend_unittest.py
@@ -6,27 +6,30 @@ import logging
 import time
 import unittest
 
-from telemetry.core.platform import factory
+from telemetry.core import platform as platform_module
 
 
 class PlatformBackendTest(unittest.TestCase):
+  def tearDown(self):
+    platform_module.ResetHostPlatformSingleton()
+
   def testPowerMonitoringSync(self):
     # Tests that the act of monitoring power doesn't blow up.
-    backend = factory.GetPlatformBackendForCurrentOS()
-    if not backend.CanMonitorPower():
+    platform = platform_module.GetHostPlatform()
+    if not platform.CanMonitorPower():
       logging.warning('Test not supported on this platform.')
       return
 
     browser_mock = lambda: None
     # Android needs to access the package of the monitored app.
-    if backend.GetOSName() == 'android':
+    if platform.GetOSName() == 'android':
       # pylint: disable=W0212
       browser_mock._browser_backend = lambda: None
       # Monitor the launcher, which is always present.
       browser_mock._browser_backend.package = 'com.android.launcher'
 
-    backend.StartMonitoringPower(browser_mock)
+    platform.StartMonitoringPower(browser_mock)
     time.sleep(0.001)
-    output = backend.StopMonitoringPower()
+    output = platform.StopMonitoringPower()
     self.assertTrue(output.has_key('power_samples_mw'))
     self.assertTrue(output.has_key('identifier'))