Adding virtualsocketserver to rtc_base sources

virtualsocketserver.h and virtualsocketserver.cc were excluded from the
sources list if build_with_chromium is true.
This causes a chromium compilation error because after [1] some deps are
explicit.

[1] - https://codereview.webrtc.org/2852663002

BUG=chromium:717832
NOTRY=True

Review-Url: https://codereview.webrtc.org/2855103003
Cr-Commit-Position: refs/heads/master@{#17989}

webrtc/base/BUILD.gn

@@ -503,6 +503,8 @@ rtc_static_library("rtc_base") {
     "stream.h",
     "thread.cc",
     "thread.h",
+    "virtualsocketserver.cc",
+    "virtualsocketserver.h",
   ]
 
   # TODO(henrike): issue 3307, make rtc_base build with the Chromium default
@@ -535,8 +537,6 @@ rtc_static_library("rtc_base") {
       "testclient.h",
       "transformadapter.cc",
       "transformadapter.h",
-      "virtualsocketserver.cc",
-      "virtualsocketserver.h",
       "window.h",
     ]