Allow PostTask() to take unique_ptr to classes derived of QueuedTask

Problem fixed by this CL: Let DerivedQueuedTask be a custom derivation of QueuedTask. Calling PostTask() with a std::unique_ptr<DerivedQueuedTask> does not work, because overload resolution sees PostTask(const Closure& closure) as a better match. The workaround of explicitly converting to std::unique_ptr<QueuedTask> before calling PostTask() results in less readable code.

Solution: Use std::enable_if to limit the template, thereby making the compiler use the right version of PostTask().

BUG=webrtc:8188

Review-Url: https://codereview.webrtc.org/3006933002
Cr-Commit-Position: refs/heads/master@{#19625}

webrtc/rtc_base/task_queue.h

@@ -14,6 +14,7 @@
 #include <list>
 #include <memory>
 #include <queue>
+#include <type_traits>
 
 #if defined(WEBRTC_MAC)
 #include <dispatch/dispatch.h>
@@ -189,7 +190,12 @@ class LOCKABLE TaskQueue {
   // more likely). This can be mitigated by limiting the use of delayed tasks.
   void PostDelayedTask(std::unique_ptr<QueuedTask> task, uint32_t milliseconds);
 
-  template <class Closure>
+  // std::enable_if is used here to make sure that calls to PostTask() with
+  // std::unique_ptr<SomeClassDerivedFromQueuedTask> would not end up being
+  // caught by this template.
+  template <class Closure,
+            typename std::enable_if<
+                std::is_copy_constructible<Closure>::value>::type* = nullptr>
   void PostTask(const Closure& closure) {
     PostTask(std::unique_ptr<QueuedTask>(new ClosureTask<Closure>(closure)));
   }