Delete SignalThread class.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index e94169d0ce45e641557d876c33bf0056e6a4e432..76d273a7e6c37551c928df2e9c64dd56e9e4527e 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -398,6 +398,7 @@ rtc_static_library("rtc_base") {
   libs = []
   defines = []
   deps = [
+    ":rtc_task_queue",
     "..:webrtc_common",
   ]
   public_deps = [
@@ -473,8 +474,6 @@ rtc_static_library("rtc_base") {
     "rtccertificate.h",
     "rtccertificategenerator.cc",
     "rtccertificategenerator.h",
-    "signalthread.cc",
-    "signalthread.h",
     "sigslot.cc",
     "sigslot.h",
     "sigslotrepeater.h",
@@ -937,7 +936,6 @@ if (rtc_include_tests) {
       "rtccertificate_unittest.cc",
       "rtccertificategenerator_unittest.cc",
       "sha1digest_unittest.cc",
-      "signalthread_unittest.cc",
       "sigslot_unittest.cc",
       "sigslottester_unittest.cc",
       "stream_unittest.cc",