PPAPI: Disallow blocking callbacks while handling a blocking message

ppapi/proxy/ppb_message_loop_proxy.cc

Index: ppapi/proxy/ppb_message_loop_proxy.cc
diff --git a/ppapi/proxy/ppb_message_loop_proxy.cc b/ppapi/proxy/ppb_message_loop_proxy.cc
index 2bc9694ab5d4deeed91c5461763a9022058916d3..555c008d523209eacd8535323d54f62b1bb6b8a1 100644
--- a/ppapi/proxy/ppb_message_loop_proxy.cc
+++ b/ppapi/proxy/ppb_message_loop_proxy.cc
@@ -31,7 +31,8 @@ MessageLoopResource::MessageLoopResource(PP_Instance instance)
       nested_invocations_(0),
       destroyed_(false),
       should_destroy_(false),
-      is_main_thread_loop_(false) {
+      is_main_thread_loop_(false),
+      currently_handling_blocking_message_(false) {
 }
 
 MessageLoopResource::MessageLoopResource(ForMainThread for_main_thread)
@@ -39,7 +40,8 @@ MessageLoopResource::MessageLoopResource(ForMainThread for_main_thread)
       nested_invocations_(0),
       destroyed_(false),
       should_destroy_(false),
-      is_main_thread_loop_(true) {
+      is_main_thread_loop_(true),
+      currently_handling_blocking_message_(false) {
   // We attach the main thread immediately. We can't use AttachToCurrentThread,
   // because the MessageLoop already exists.
 
@@ -193,6 +195,10 @@ base::MessageLoopProxy* MessageLoopResource::GetMessageLoopProxy() {
   return loop_proxy_.get();
 }
 
+bool MessageLoopResource::CurrentlyHandlingBlockingMessage() {
+  return currently_handling_blocking_message_;
+}
+
 // static
 void MessageLoopResource::ReleaseMessageLoop(void* value) {
   static_cast<MessageLoopResource*>(value)->DetachFromThread();