Bug 1436279 - Remove unneeded mutable from DisplayItemClipChain::mRefCount - r=mattwoodrow

mRefCount is only accessed from non-const methods, so there is no need for
'mutable'.

MozReview-Commit-ID: Bwv6cO1utU5

--HG--
extra : rebase_source : a7851c9863ca9aeff1deaff4408b89ee1b9f8445

layout/painting/DisplayItemClipChain.h

@@ -80,7 +80,7 @@ struct DisplayItemClipChain {
   DisplayItemClip mClip;
   const ActiveScrolledRoot* mASR;
   RefPtr<const DisplayItemClipChain> mParent;
-  mutable uint32_t mRefCount = 0;
+  uint32_t mRefCount = 0;
 #ifdef DEBUG
   bool mOnStack;
 #endif