Bug 1062533 - part 5 - add {Mutex,Monitor}::TryLock methods; r=mccr8

This plumbs the code from PlatformMutex up to the xpcom level.

xpcom/threads/BlockingResourceBase.cpp

@@ -349,6 +349,16 @@ void OffTheBooksMutex::Lock() {
   Acquire();
 }
 
+bool OffTheBooksMutex::TryLock() {
+  CheckAcquire();
+  bool locked = this->tryLock();
+  if (locked) {
+    mOwningThread = PR_GetCurrentThread();
+    Acquire();
+  }
+  return locked;
+}
+
 void OffTheBooksMutex::Unlock() {
   Release();
   mOwningThread = nullptr;

xpcom/threads/Monitor.h

@@ -30,6 +30,7 @@ class Monitor {
   ~Monitor() {}
 
   void Lock() { mMutex.Lock(); }
+  bool TryLock() { return mMutex.TryLock(); }
   void Unlock() { mMutex.Unlock(); }
 
   void Wait() { mCondVar.Wait(); }

xpcom/threads/Mutex.h

@@ -65,6 +65,11 @@ class OffTheBooksMutex : public detail::MutexImpl, BlockingResourceBase {
    **/
   void Lock() { this->lock(); }
 
+  /**
+   * Try to lock this mutex, returning true if we were successful.
+   **/
+  bool TryLock() { return this->tryLock(); }
+
   /**
    * Unlock this mutex.
    **/
@@ -89,6 +94,7 @@ class OffTheBooksMutex : public detail::MutexImpl, BlockingResourceBase {
 
 #else
   void Lock();
+  bool TryLock();
   void Unlock();
 
   void AssertCurrentThreadOwns() const;