When killing wptserve, use Executive.interrupt on non-Windows platforms.

third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py

Index: third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py
diff --git a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py
index 13c8edf3a2b887fd8ec147ff5a28eb08412c562f..0fdc2c8ecf17b200794cc90999f61142150187ee 100644
--- a/third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py
+++ b/third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_mock.py
@@ -152,6 +152,9 @@ class MockExecutive(object):
     def kill_process(self, pid):
         pass
 
+    def interrupt(self, pid):
+        pass
+
     def popen(self, args, cwd=None, env=None, **_):
         assert all(isinstance(arg, basestring) for arg in args)
         self.calls.append(args)