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 c476a12e8a3f8e629f184c4c819b9f1288a19904..66220eb72b785c64ba7f158622d4999fe84d846a 100644
--- a/webrtc/test/BUILD.gn
+++ b/webrtc/test/BUILD.gn
@@ -128,6 +128,8 @@ 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",
@@ -148,6 +150,7 @@ rtc_source_set("test_support") {
     "../system_wrappers",
     "//testing/gmock",
     "//testing/gtest",
+    "//third_party/gflags",
   ]
 
   public_deps = [
@@ -213,6 +216,7 @@ 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",
@@ -248,6 +252,7 @@ rtc_test("test_support_unittests") {
     "../modules/video_capture",
     "//testing/gmock",
     "//testing/gtest",
+    "//third_party/gflags",
   ]
 }
 rtc_source_set("fileutils_unittests") {