Improve detection of crashes and hangs with WebDriver.

This makes crash report work with WebKitWebDriver:
 * When the WebKitWebProcess crashes, WebKitWebDriver will raise an
   InvalidSessionIdException <https://webkit.org/b/207565> like the
   Chrome WebDriver does. So we make WebDriverProtocol::is_alive()
   check for that exception.

It also fixes detecting when the WebDriver has hanged:
 * The call to is_alive() can hang if WebDriver itself has hanged
   or its unresponsive. If this call takes more than 5 seconds then
   the external timeout from the testrunner will be fired and testrunner
   will force a termination of the test marking it as external-timeout
   instead of crash.
 * The current logic tried to mark the test as crashing if socket
   timeout fired, but that timeout value was much higher than the
   grace period of 5 seconds, so this didn't worked.
 * We modify the socket timeout to 2 seconds, which should be more
   than enough to check if the WebDriver its still alive and allows
   to complete the check during the testrunner 5 seconds of
   extra_timeout.

tools/wptrunner/wptrunner/executors/base.py

@@ -163,10 +163,14 @@ def run(self):
                 self.result = False, ("INTERNAL-ERROR", "%s.run_func didn't set a result" %
                                       self.__class__.__name__)
             else:
-                message = "Executor hit external timeout (this may indicate a hang)\n"
-                # get a traceback for the current stack of the executor thread
-                message += "".join(traceback.format_stack(sys._current_frames()[executor.ident]))
-                self.result = False, ("EXTERNAL-TIMEOUT", message)
+                if self.protocol.is_alive():
+                    message = "Executor hit external timeout (this may indicate a hang)\n"
+                    # get a traceback for the current stack of the executor thread
+                    message += "".join(traceback.format_stack(sys._current_frames()[executor.ident]))
+                    self.result = False, ("EXTERNAL-TIMEOUT", message)
+                else:
+                    self.logger.info("Browser not responding, setting status to CRASH")
+                    self.result = False, ("CRASH", None)
         elif self.result[1] is None:
             # We didn't get any data back from the test, so check if the
             # browser is still responsive

tools/wptrunner/wptrunner/executors/executorwebdriver.py

@@ -301,11 +301,19 @@ def teardown(self):
         self.webdriver = None
 
     def is_alive(self):
+        socket_previous_timeout = socket.getdefaulttimeout()
         try:
-            # Get a simple property over the connection
+            # Get a simple property over the connection, with 2 seconds of timeout
+            # that should be more than enough to check if the WebDriver its
+            # still alive, and allows to complete the check within the testrunner
+            # 5 seconds of extra_timeout we have as maximum to end the test before
+            # the external timeout from testrunner triggers.
+            socket.setdefaulttimeout(2)
             self.webdriver.window_handle
-        except (socket.timeout, client.UnknownErrorException):
+        except (socket.timeout, client.UnknownErrorException, client.InvalidSessionIdException):
             return False
+        finally:
+            socket.setdefaulttimeout(socket_previous_timeout)
         return True
 
     def after_connect(self):