Revert of Delete unused TaskRunner abstraction.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index cd17fa60972ecbd6fad1479a7e716af3446d03d5..d8f5e2b0964b9e36307b5d493fe35cd62390c757 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -346,6 +346,19 @@
       "-Wno-missing-braces",
     ]
   }
+}
+
+# TODO(nisse): This target is temporarily split out, to aid moving the
+# code over to Chrome. It should be deleted from webrtc soon.
+rtc_static_library("rtc_task_runner") {
+  sources = [
+    "task.cc",
+    "task.h",
+    "taskparent.cc",
+    "taskparent.h",
+    "taskrunner.cc",
+    "taskrunner.h",
+  ]
 }
 
 rtc_static_library("rtc_base") {
@@ -857,6 +870,7 @@
       "sigslot_unittest.cc",
       "sigslottester_unittest.cc",
       "stream_unittest.cc",
+      "task_unittest.cc",
       "testclient_unittest.cc",
       "thread_unittest.cc",
     ]
@@ -878,6 +892,7 @@
     }
     deps = [
       ":rtc_base_tests_main",
+      ":rtc_task_runner",
     ]
     public_deps = [
       ":rtc_base",