Reset accessibility if it gets out of sync.

content/common/accessibility_messages.h

Index: content/common/accessibility_messages.h
diff --git a/content/common/accessibility_messages.h b/content/common/accessibility_messages.h
index 02af80708cce2f135c17b19427b939521a71fab6..5abf149303d45bf183ef250ee99abae830096660 100644
--- a/content/common/accessibility_messages.h
+++ b/content/common/accessibility_messages.h
@@ -131,16 +131,32 @@ IPC_MESSAGE_ROUTED1(AccessibilityMsg_HitTest,
 // message was processed and it can send addition events.
 IPC_MESSAGE_ROUTED0(AccessibilityMsg_Events_ACK)
 
-// Kill the renderer because we got a fatal error in the accessibility tree.
+// Tell the renderer to reset and send a new accessibility tree from
+// scratch because the browser is out of sync. It passes a sequential
+// reset token. This should be rare, and if we need reset the same renderer
+// too many times we just kill it. After sending a reset, the browser ignores
+// incoming accessibility IPCs until it receives one with the matching reset
+// token. Conversely, it ignores IPCs with a reset token if it was not
+// expecting a reset.
+IPC_MESSAGE_ROUTED1(AccessibilityMsg_Reset,
+                    int /* reset token */);
+
+// Kill the renderer because we got a fatal error in the accessibility tree
+// and we've already reset too many times.
 IPC_MESSAGE_ROUTED0(AccessibilityMsg_FatalError)
 
 // Messages sent from the renderer to the browser.
 
 // Sent to notify the browser about renderer accessibility events.
 // The browser responds with a AccessibilityMsg_Events_ACK.
-IPC_MESSAGE_ROUTED1(
+// The second parameter, reset_token, is set if this IPC was sent in response
+// to a reset request from the browser. When the browser requests a reset,
+// it ignores incoming IPCs until it sees one with the correct reset token.
+// Any other time, it ignores IPCs with a reset token.
+IPC_MESSAGE_ROUTED2(
     AccessibilityHostMsg_Events,
-    std::vector<AccessibilityHostMsg_EventParams>)
+    std::vector<AccessibilityHostMsg_EventParams> /* events */,
+    int /* reset_token */)
 
 // Sent to update the browser of the location of accessibility objects.
 IPC_MESSAGE_ROUTED1(