Revert of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3004373002/ )

Reason for revert:
It breaks a chromium FYI trybot (the switch from rtc_static_library to rtc_source_set seems to be the problem).

Original issue's description:
> Reland of Decoupling rtc_base from Obj-C code (patchset #1 id:1 of https://codereview.webrtc.org/3008103002/ )
>
> Reason for revert:
> Trying to reland after fixing downstream project.
>
> Original issue's description:
> > Revert of Decoupling rtc_base from Obj-C code (patchset #8 id:140001 of https://codereview.webrtc.org/3001623003/ )
> >
> > Reason for revert:
> > It breaks a downstream project.
> >
> > Original issue's description:
> > > Decoupling rtc_base from Obj-C code
> > >
> > > The goal of this CL is to separate Obj-C/Obj-C++ code from targets which have
> > > also C++ code (see https://bugs.chromium.org/p/webrtc/issues/detail?id=7743
> > > for more information).
> > >
> > > To achieve this we have created 2 targets (rtc_base_objc and rtc_base_generic)
> > > and rtc_base will act as a proxy between these targets (this way we can avoid a
> > > circular dependency between rtc_base_generic and rtc_base_objc).
> > >
> > > BUG=webrtc:7743
> > >
> > > Review-Url: https://codereview.webrtc.org/3001623003
> > > Cr-Commit-Position: refs/heads/master@{#19661}
> > > Committed: https://chromium.googlesource.com/external/webrtc/+/a0a5b98937de0f0bf90e17011d2cbc8c632cb409
> >
> > TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> > # Skipping CQ checks because original CL landed less than 1 days ago.
> > NOPRESUBMIT=true
> > NOTREECHECKS=true
> > NOTRY=true
> > BUG=webrtc:7743
> >
> > Review-Url: https://codereview.webrtc.org/3008103002
> > Cr-Commit-Position: refs/heads/master@{#19664}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/791a8f611da8cc6b0668ccc03bf7525e6ba72a46
>
> TBR=kjellander@webrtc.org,kwiberg@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:7743
>
> Review-Url: https://codereview.webrtc.org/3004373002
> Cr-Commit-Position: refs/heads/master@{#19681}
> Committed: https://chromium.googlesource.com/external/webrtc/+/a0c893f324da5c9f0773f8990a29cd24643864e4

TBR=kjellander@webrtc.org,kwiberg@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/3007183002
Cr-Original-Commit-Position: refs/heads/master@{#19688}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 9c5e511806ef64c4cede52722c8a7ae7ae50ff56
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 3cb7766..1914b27 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -429,28 +429,7 @@
   }
 }
 
-rtc_source_set("rtc_base") {
-  public_deps = [
-    ":rtc_base_generic",
-  ]
-  if (is_ios || is_mac) {
-    public_deps += [ ":rtc_base_objc" ]
-  }
-}
-
-if (is_ios || is_mac) {
-  rtc_source_set("rtc_base_objc") {
-    sources = [
-      "thread_darwin.mm",
-    ]
-    deps = [
-      ":rtc_base_generic",
-    ]
-    visibility = [ ":rtc_base" ]
-  }
-}
-
-rtc_source_set("rtc_base_generic") {
+rtc_static_library("rtc_base") {
   cflags = []
   cflags_cc = []
   libs = []
@@ -559,11 +538,6 @@
     "thread.h",
   ]
 
-  visibility = [
-    ":rtc_base",
-    ":rtc_base_objc",
-  ]
-
   # TODO(henrike): issue 3307, make rtc_base build with the Chromium default
   # compiler settings.
   suppressed_configs += [ "//build/config/compiler:chromium_code" ]
@@ -623,7 +597,10 @@
   }
 
   if (is_ios || is_mac) {
-    sources += [ "macifaddrs_converter.cc" ]
+    sources += [
+      "macifaddrs_converter.cc",
+      "thread_darwin.mm",
+    ]
   }
 
   if (use_x11) {