Fix BUILD.gn to incorporate more existing files.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 787ebd9f137cc635efc91e8eb4c19f506a65162d..d223f1341222a60036622361d52085365fbd0807 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -72,6 +72,7 @@ source_set("gpr") {
     "src/core/lib/support/backoff.h",
     "src/core/lib/support/block_annotate.h",
     "src/core/lib/support/env.h",
+    "src/core/lib/support/mpscq.h",
     "src/core/lib/support/murmur_hash.h",
     "src/core/lib/support/percent_encoding.h",
     "src/core/lib/support/stack_lockfree.h",
@@ -100,6 +101,7 @@ source_set("gpr") {
     "src/core/lib/support/log_linux.c",
     "src/core/lib/support/log_posix.c",
     "src/core/lib/support/log_windows.c",
+    "src/core/lib/support/mpscq.c",
     "src/core/lib/support/murmur_hash.c",
     "src/core/lib/support/percent_encoding.c",
     "src/core/lib/support/slice.c",
@@ -188,6 +190,7 @@ source_set("grpc") {
     "src/core/lib/http/httpcli.h",
     "src/core/lib/http/parser.h",
     "src/core/lib/iomgr/closure.h",
+    "src/core/lib/iomgr/combiner.h",
     "src/core/lib/iomgr/endpoint.h",
     "src/core/lib/iomgr/endpoint_pair.h",
     "src/core/lib/iomgr/error.h",
@@ -330,6 +333,7 @@ source_set("grpc") {
     "src/core/lib/http/httpcli.c",
     "src/core/lib/http/parser.c",
     "src/core/lib/iomgr/closure.c",
+    "src/core/lib/iomgr/combiner.c",
     "src/core/lib/iomgr/endpoint.c",
     "src/core/lib/iomgr/endpoint_pair_posix.c",
     "src/core/lib/iomgr/endpoint_pair_windows.c",
@@ -550,6 +554,7 @@ source_set("grpc_cronet") {
     "src/core/lib/http/httpcli.h",
     "src/core/lib/http/parser.h",
     "src/core/lib/iomgr/closure.h",
+    "src/core/lib/iomgr/combiner.h",
     "src/core/lib/iomgr/endpoint.h",
     "src/core/lib/iomgr/endpoint_pair.h",
     "src/core/lib/iomgr/error.h",
@@ -691,6 +696,7 @@ source_set("grpc_cronet") {
     "src/core/lib/http/httpcli.c",
     "src/core/lib/http/parser.c",
     "src/core/lib/iomgr/closure.c",
+    "src/core/lib/iomgr/combiner.c",
     "src/core/lib/iomgr/endpoint.c",
     "src/core/lib/iomgr/endpoint_pair_posix.c",
     "src/core/lib/iomgr/endpoint_pair_windows.c",
@@ -901,6 +907,7 @@ source_set("grpc_unsecure") {
     "src/core/lib/http/httpcli.h",
     "src/core/lib/http/parser.h",
     "src/core/lib/iomgr/closure.h",
+    "src/core/lib/iomgr/combiner.h",
     "src/core/lib/iomgr/endpoint.h",
     "src/core/lib/iomgr/endpoint_pair.h",
     "src/core/lib/iomgr/error.h",
@@ -1020,6 +1027,7 @@ source_set("grpc_unsecure") {
     "src/core/lib/http/httpcli.c",
     "src/core/lib/http/parser.c",
     "src/core/lib/iomgr/closure.c",
+    "src/core/lib/iomgr/combiner.c",
     "src/core/lib/iomgr/endpoint.c",
     "src/core/lib/iomgr/endpoint_pair_posix.c",
     "src/core/lib/iomgr/endpoint_pair_windows.c",