Prevent Client::Tick() from causing reentrancy in the plugin on Mac OS X....

plugin/mac/plugin_mac.mm

Index: plugin/mac/plugin_mac.mm
===================================================================
--- plugin/mac/plugin_mac.mm	(revision 48764)
+++ plugin/mac/plugin_mac.mm	(working copy)
@@ -237,9 +237,10 @@
     NPP instance = instances_[i];
     PluginObject* obj = static_cast<PluginObject*>(instance->pdata);
 
-    // RenderClient() may cause events to be processed, leading to
-    // reentrant calling of this code. Detect and avoid this case.
-    if (obj->client()->IsRendering()) {
+    // RenderClient() and Tick() may cause events to be processed,
+    // leading to reentrant calling of this code. Detect and avoid
+    // this case.
+    if (obj->client()->IsRendering() || obj->client()->IsTicking()) {
       continue;
     }