Revert "Revert "Implement call combiner""

This reverts commit bf19961d0a49b43cb528392efeb4880eeebb9b5e.
diff --git a/BUILD b/BUILD
index 67e727b..b8062e4 100644
--- a/BUILD
+++ b/BUILD
@@ -577,6 +577,7 @@
         "src/core/lib/http/format_request.c",
         "src/core/lib/http/httpcli.c",
         "src/core/lib/http/parser.c",
+        "src/core/lib/iomgr/call_combiner.c",
         "src/core/lib/iomgr/closure.c",
         "src/core/lib/iomgr/combiner.c",
         "src/core/lib/iomgr/endpoint.c",
@@ -709,6 +710,7 @@
         "src/core/lib/http/format_request.h",
         "src/core/lib/http/httpcli.h",
         "src/core/lib/http/parser.h",
+        "src/core/lib/iomgr/call_combiner.h",
         "src/core/lib/iomgr/closure.h",
         "src/core/lib/iomgr/combiner.h",
         "src/core/lib/iomgr/endpoint.h",