[blinkpy] Apply /t to taskkill based on kill_tree

This was likely missed in https://crrev.com/c/1769846. With this change,
kill_tree now works on Windows and Linux in the same way.

Change-Id: Ib5de5bff3ab934846e46ee0e9210d5be5f308dc2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462197Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815590}

third_party/blink/tools/blinkpy/common/system/executive.py

@@ -126,7 +126,10 @@ class Executive(object):
                 NtSuspendProcess(process_handle)
                 CloseHandle(process_handle)
 
-            command = ['taskkill.exe', '/f', '/t', '/pid', pid]
+            command = ['taskkill.exe', '/f']
+            if kill_tree:
+                command.append('/t')
+            command += ['/pid', pid]
             # taskkill will exit 128 if the process is not found. We should log.
             self.run_command(command, error_handler=self.log_error)
             return