Fixing potential AsyncInvoker deadlock that occurs for "reentrant" invocations.

The deadlock occurs if the AsyncInvoker is destroyed on thread A while
a task on thread B is running, which AsyncInvokes a task back on thread
A.

This was causing pending_invocations_ to end up negative, because
an AsyncClosure that's never added to a thread's message queue (due to
the "destroying_" flag) caused the count to be decremented but not
incremented.

BUG=None

Review-Url: https://codereview.webrtc.org/2885143006
Cr-Commit-Position: refs/heads/master@{#18225}

webrtc/base/asyncinvoker.h

@@ -120,7 +120,7 @@ class AsyncInvoker : public MessageHandler {
                        uint32_t id);
   volatile int pending_invocations_ = 0;
   Event invocation_complete_;
-  bool destroying_ = false;
+  int destroying_ = 0;
   friend class AsyncClosure;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(AsyncInvoker);