Bug 1322435. Limit the number of image decoding threads we use in the parent process. r=aosmond

There aren't as many images in chrome to decode, and they aren't as big, so we don't need as many threads.

image/DecodePool.cpp

@@ -16,6 +16,7 @@
 #include "nsThreadUtils.h"
 #include "nsXPCOMCIDInternal.h"
 #include "prsystem.h"
+#include "nsIXULRuntime.h"
 
 #include "gfxPrefs.h"
 
@@ -261,6 +262,11 @@ DecodePool::DecodePool()
   if (limit > 32) {
     limit = 32;
   }
+  // The parent process where there are content processes doesn't need as many
+  // threads for decoding images.
+  if (limit > 4 && XRE_IsParentProcess() && BrowserTabsRemoteAutostart()) {
+    limit = 4;
+  }
 
   // Initialize the thread pool.
   for (uint32_t i = 0 ; i < limit ; ++i) {