Bug 855923 - Set the multithreaded image decoding thread pool's idle thread limit in addition to the regular thread limit. r=seth

image/src/RasterImage.cpp

@@ -3522,12 +3522,16 @@ RasterImage::DecodePool::DecodePool()
     mThreadPool = do_CreateInstance(NS_THREADPOOL_CONTRACTID);
     if (mThreadPool) {
       mThreadPool->SetName(NS_LITERAL_CSTRING("ImageDecoder"));
+      uint32_t limit;
       if (gDecodingThreadLimit <= 0) {
-        mThreadPool->SetThreadLimit(std::max(PR_GetNumberOfProcessors() - 1, 1));
+        limit = std::max(PR_GetNumberOfProcessors(), 2) - 1;
       } else {
-        mThreadPool->SetThreadLimit(static_cast<uint32_t>(gDecodingThreadLimit));
+        limit = static_cast<uint32_t>(gDecodingThreadLimit);
       }
 
+      mThreadPool->SetThreadLimit(limit);
+      mThreadPool->SetIdleThreadLimit(limit);
+
       nsCOMPtr<nsIObserverService> obsSvc = mozilla::services::GetObserverService();
       if (obsSvc) {
         obsSvc->AddObserver(this, "xpcom-shutdown-threads", false);