Add gn dependency on rtc_task_runner.

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index addf59f4690021ee3e1e7631fd56a75b6a214a5c..bfeec449ccc9e730c0b6776403fc2f02f223af6c 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -331,6 +331,7 @@ static_library("test_support") {
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",
       "//third_party/webrtc/base:rtc_base",
+      "//third_party/webrtc/base:rtc_task_runner",
       "//third_party/webrtc/media:rtc_media_base",
       "//third_party/webrtc/modules/video_capture",
       "//third_party/webrtc/stats:rtc_stats",
@@ -1610,6 +1611,7 @@ test("content_unittests") {
       "//third_party/webrtc/api:libjingle_peerconnection",
       "//third_party/webrtc/api:rtc_stats_api",
       "//third_party/webrtc/base:rtc_base",
+      "//third_party/webrtc/base:rtc_task_runner",
       "//third_party/webrtc/media:rtc_media",
       "//third_party/webrtc/modules/desktop_capture:primitives",
       "//third_party/webrtc/modules/video_capture",