Fix recursion in handling touch input events acks.

content/browser/renderer_host/input/passthrough_touch_event_queue.cc

Index: content/browser/renderer_host/input/passthrough_touch_event_queue.cc
diff --git a/content/browser/renderer_host/input/passthrough_touch_event_queue.cc b/content/browser/renderer_host/input/passthrough_touch_event_queue.cc
index a13ccbf51dfc60c013d843d4c448ec8b0b734877..b9cc039df19a5282ef8f436436ea6276e7c92afd 100644
--- a/content/browser/renderer_host/input/passthrough_touch_event_queue.cc
+++ b/content/browser/renderer_host/input/passthrough_touch_event_queue.cc
@@ -117,7 +117,8 @@ PassthroughTouchEventQueue::PassthroughTouchEventQueue(
       maybe_has_handler_for_current_sequence_(false),
       drop_remaining_touches_in_sequence_(false),
       touchmove_slop_suppressor_(new TouchMoveSlopSuppressor),
-      send_touch_events_async_(false) {
+      send_touch_events_async_(false),
+      processing_acks_(false) {
   if (config.touch_ack_timeout_supported) {
     timeout_handler_.reset(
         new TouchTimeoutHandler(this, config.desktop_touch_ack_timeout_delay,
@@ -265,6 +266,11 @@ void PassthroughTouchEventQueue::FlushQueue() {
 }
 
 void PassthroughTouchEventQueue::AckCompletedEvents() {
+  // Don't allow re-entrancy into this method otherwise
+  // the ordering of acks won't be preserved.
+  if (processing_acks_)
+    return;
+  base::AutoReset<bool> process_acks(&processing_acks_, true);
   while (!outstanding_touches_.empty()) {
     auto iter = outstanding_touches_.begin();
     if (iter->ack_state() == INPUT_EVENT_ACK_STATE_UNKNOWN)