Delete SignalThread class.

webrtc/rtc_base/BUILD.gn

Index: webrtc/rtc_base/BUILD.gn
diff --git a/webrtc/rtc_base/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index 851973b15a07cb06ac9a8f6bb7604650c8be81c0..9e905820d26557d59fc52bc0188b0e0d36034067 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -404,6 +404,7 @@ rtc_static_library("rtc_base") {
   libs = []
   defines = []
   deps = [
+    ":rtc_task_queue",
     "..:webrtc_common",
   ]
   public_deps = [
@@ -479,8 +480,6 @@ rtc_static_library("rtc_base") {
     "rtccertificate.h",
     "rtccertificategenerator.cc",
     "rtccertificategenerator.h",
-    "signalthread.cc",
-    "signalthread.h",
     "sigslot.cc",
     "sigslot.h",
     "socket.h",
@@ -989,7 +988,6 @@ 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",