Shut down browser processes on Windows by sending WM_CLOSE messages.

tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py

Index: tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py
diff --git a/tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py b/tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py
index e4e0ff2bbbb15c44ea74196bf128bd57f4f93e09..b7bfb5ade81539383ec75f9f9da51f08b3361b2f 100644
--- a/tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py
+++ b/tools/telemetry/telemetry/core/backends/chrome/desktop_browser_backend.py
@@ -326,9 +326,28 @@ class DesktopBrowserBackend(chrome_browser_backend.ChromeBrowserBackend):
   def __del__(self):
     self.Close()
 
+  def _TryCooperativeShutdown(self):
+    if self.browser.platform.IsCooperativeShutdownSupported():
+      # Ideally there would be a portable, cooperative shutdown
+      # mechanism for the browser. This seems difficult to do
+      # correctly for all embedders of the content API. The only known
+      # problem with unclean shutdown of the browser process is on
+      # Windows, where suspended child processes frequently leak. For
+      # now, just solve this particular problem. See Issue 424024.
+      if self.browser.platform.CooperativelyShutdown(self._proc, "chrome"):
+        try:
+          util.WaitFor(lambda: not self.IsBrowserRunning(), timeout=5)
+          logging.info('Successfully shut down browser cooperatively')
+        except util.TimeoutException as e:
+          logging.warning('Failed to cooperatively shutdown. ' +
+                          'Proceeding to terminate: ' + str(e))
+
   def Close(self):
     super(DesktopBrowserBackend, self).Close()
 
+    if self.IsBrowserRunning():
+      self._TryCooperativeShutdown()
+
     # Shutdown politely if the profile may be used again.
     if self._output_profile_path and self.IsBrowserRunning():
       self._proc.terminate()