Revert "Revert "Implement call combiner""

This reverts commit bf19961d0a49b43cb528392efeb4880eeebb9b5e.
diff --git a/grpc.gyp b/grpc.gyp
index 40938a4..5c4510a 100644
--- a/grpc.gyp
+++ b/grpc.gyp
@@ -238,6 +238,7 @@
         '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',
@@ -538,6 +539,7 @@
         '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',
@@ -743,6 +745,7 @@
         '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',
@@ -933,6 +936,7 @@
         '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',