Add WriteIsolatedOutput() functions

webrtc/test/BUILD.gn

Index: webrtc/test/BUILD.gn
diff --git a/webrtc/test/BUILD.gn b/webrtc/test/BUILD.gn
index 22f890b64fbf53d63f3042d4fb7df424dd3183b7..ab68496174b6ac0290a0688681a04b0c944b84a6 100644
--- a/webrtc/test/BUILD.gn
+++ b/webrtc/test/BUILD.gn
@@ -126,6 +126,8 @@ rtc_source_set("test_support") {
     "testsupport/frame_reader.h",
     "testsupport/frame_writer.cc",
     "testsupport/frame_writer.h",
+    "testsupport/isolated_output.cc",
+    "testsupport/isolated_output.h",
     "testsupport/metrics/video_metrics.cc",
     "testsupport/metrics/video_metrics.h",
     "testsupport/mock/mock_frame_reader.h",
@@ -146,6 +148,7 @@ rtc_source_set("test_support") {
     "../system_wrappers",
     "//testing/gmock",
     "//testing/gtest",
+    "//third_party/gflags",
   ]
 
   public_deps = [
@@ -211,6 +214,7 @@ rtc_test("test_support_unittests") {
     "testsupport/always_passing_unittest.cc",
     "testsupport/frame_reader_unittest.cc",
     "testsupport/frame_writer_unittest.cc",
+    "testsupport/isolated_output_unittest.cc",
     "testsupport/metrics/video_metrics_unittest.cc",
     "testsupport/packet_reader_unittest.cc",
     "testsupport/perf_test_unittest.cc",
@@ -246,6 +250,7 @@ rtc_test("test_support_unittests") {
     "../modules/video_capture",
     "//testing/gmock",
     "//testing/gtest",
+    "//third_party/gflags",
   ]
 }
 rtc_source_set("fileutils_unittests") {