Revert of Delete SignalThread class. (patchset #20 id:380001 of https://codereview.webrtc.org/2915253002/ )

Reason for revert:
Seems to be causing new crashes, possibly because of changes to the "Destroy(false)" behavior. Will re-land after investigating these crashes more and addressing the root cause.

Original issue's description:
> Delete SignalThread class.
>
> Rewrite AsyncResolver to use PlatformThread directly, not
> SignalThread, and update includes of peerconnection client to not
> depend on signalthread.h.
>
> BUG=webrtc:6424,webrtc:7723
>
> Review-Url: https://codereview.webrtc.org/2915253002
> Cr-Commit-Position: refs/heads/master@{#18833}
> Committed: https://chromium.googlesource.com/external/webrtc/+/bc8feda1db02b2a9b501e4aa43926ca7e861b638

TBR=tommi@webrtc.org,kwiberg@webrtc.org,nisse@webrtc.org
NOPRESUBMIT=true
NOTRY=true
BUG=webrtc:6424,webrtc:7723

Review-Url: https://codereview.webrtc.org/2979733002
Cr-Commit-Position: refs/heads/master@{#18980}
diff --git a/webrtc/rtc_base/BUILD.gn b/webrtc/rtc_base/BUILD.gn
index 9e90582..851973b 100644
--- a/webrtc/rtc_base/BUILD.gn
+++ b/webrtc/rtc_base/BUILD.gn
@@ -404,7 +404,6 @@
   libs = []
   defines = []
   deps = [
-    ":rtc_task_queue",
     "..:webrtc_common",
   ]
   public_deps = [
@@ -480,6 +479,8 @@
     "rtccertificate.h",
     "rtccertificategenerator.cc",
     "rtccertificategenerator.h",
+    "signalthread.cc",
+    "signalthread.h",
     "sigslot.cc",
     "sigslot.h",
     "socket.h",
@@ -988,6 +989,7 @@
       "rtccertificate_unittest.cc",
       "rtccertificategenerator_unittest.cc",
       "sha1digest_unittest.cc",
+      "signalthread_unittest.cc",
       "sigslot_unittest.cc",
       "sigslottester_unittest.cc",
       "stream_unittest.cc",