Shotdown browser cooperatively with taskill on win platform.

tools/telemetry/telemetry/core/platform/platform_backend.py

Index: tools/telemetry/telemetry/core/platform/platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/platform_backend.py b/tools/telemetry/telemetry/core/platform/platform_backend.py
index 4264f035ea1ccfb61ed8195f3769e9d3b33d36d0..6fa1b23d811dcb82ebc40e02ffa50c21ca382115 100644
--- a/tools/telemetry/telemetry/core/platform/platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/platform_backend.py
@@ -123,11 +123,11 @@ class PlatformBackend(object):
     # tied to the browser. https://crbug.com/424777
     self._network_controller_backend.StopReplay()
 
-    is_last_browser = len(self._running_browser_backends) == 1
+    is_last_browser = len(self._running_browser_backends) <= 1
     if is_last_browser:
       self.SetFullPerformanceModeEnabled(False)
 
-    self._running_browser_backends.remove(browser_backend)
+    self._running_browser_backends.discard(browser_backend)
 
   def GetBackendForBrowser(self, browser):
     matches = [x for x in self._running_browser_backends
@@ -264,3 +264,16 @@ class PlatformBackend(object):
   @property
   def wpr_ca_cert_path(self):
     return None
+
+  def IsProcessRunning(self, proc):
+    return proc and proc.poll() == None
+
+  def IsCooperativeShutdownSupported(self):
+    return False
+
+  def CooperativelyShutdown(self, proc):
+    """Cooperatively shut down the given process from subprocess.Popen.
+    Returns True if it is believed the attempt succeeded.
+
+    Currently this is only implemented on Windows."""
+    raise NotImplementedError()