Make telemetry platform a singleton

tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py

Index: tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py
diff --git a/tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py b/tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py
index 2908e7ecc80892c33dc36d847c3a83e204ac500c..cac36dc2cf0f149dcb8ccc754e6d36add0bb3996 100644
--- a/tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py
+++ b/tools/telemetry/telemetry/core/platform/posix_platform_backend_unittest.py
@@ -6,8 +6,8 @@ import sys
 import unittest
 
 from telemetry import benchmark
-from telemetry.core.platform import factory
 from telemetry.core.platform import posix_platform_backend
+from telemetry.core import platform as platform_module
 
 
 class TestBackend(posix_platform_backend.PosixPlatformBackend):
@@ -26,6 +26,8 @@ class TestBackend(posix_platform_backend.PosixPlatformBackend):
 
 
 class PosixPlatformBackendTest(unittest.TestCase):
+  def tearDown(self):
+    platform_module.ResetHostPlatformSingleton()
 
   def testGetChildPidsWithGrandChildren(self):
     backend = TestBackend()
@@ -53,10 +55,10 @@ class PosixPlatformBackendTest(unittest.TestCase):
 
   @benchmark.Enabled('linux', 'mac')
   def testIsApplicationRunning(self):
-    backend = factory.GetPlatformBackendForCurrentOS()
+    platform = platform_module.GetHostPlatform()
 
-    self.assertFalse(backend.IsApplicationRunning('This_Is_A_Bad___App__Name'))
+    self.assertFalse(platform.IsApplicationRunning('This_Is_A_Bad___App__Name'))
     sys_exe = os.path.basename(sys.executable)
-    self.assertTrue(backend.IsApplicationRunning(sys_exe))
+    self.assertTrue(platform.IsApplicationRunning(sys_exe))
     self.assertFalse(
-        backend.IsApplicationRunning('%s append_bad_after_space' % sys_exe))
+        platform.IsApplicationRunning('%s append_bad_after_space' % sys_exe))