SurfaceFlinger: Add ClientCache dumps

Can be useful in debugging buffer leaks.

Bug: 176691195
Test: Existing tests pass
Change-Id: I8b318f272a7da3dde76d9153b21ced94127f0d8b
diff --git a/services/surfaceflinger/ClientCache.h b/services/surfaceflinger/ClientCache.h
index d7af7c0..0d597c8 100644
--- a/services/surfaceflinger/ClientCache.h
+++ b/services/surfaceflinger/ClientCache.h
@@ -53,6 +53,8 @@
     void unregisterErasedRecipient(const client_cache_t& cacheId,
                                    const wp<ErasedRecipient>& recipient);
 
+    void dump(std::string& result);
+
 private:
     std::mutex mMutex;