QSampleCache: Add a method to check whether a sample is cached.

At the moment there is no way to tell whether a sample
returned by QSampleCache comes from cache.
This functionality is required for a unit test.

Change-Id: Ice23bb39b8e4f0a58b9297b46bb19b9a8c34b53c
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>

src/multimedia/audio/qsamplecache_p.cpp

@@ -147,6 +147,12 @@ bool QSampleCache::isLoading() const
     return m_loadingThread.isRunning();
 }
 
+bool QSampleCache::isCached(const QUrl &url) const
+{
+    QMutexLocker locker(&m_mutex);
+    return m_samples.contains(url);
+}
+
 QSample* QSampleCache::requestSample(const QUrl& url)
 {
     //lock and add first to make sure live loadingThread will not be killed during this function call