Revert of Fix inconsistent setting of the rtc_include_unittests flag. (patchset #1 id:1 of https://codereview.webrtc.org/2154693002/ )

Reason for revert:
Nope, breaks chromium

Original issue's description:
> Fix inconsistent setting of the rtc_include_unittests flag.
>
> On advice from brettw, se discussion here:
> https://codereview.webrtc.org/2149543002/
>
> The problem was we were setting the flag to both false and true,
> and the the true happened to win out for WebRTC checkouts and
> false for Chromium checkouts. This change should make this
> mechanic more obvious.
>
> This change _should_ have no effect downstream.
>
> Doing tbr to see if we can get the chromium import back into a good state today.
>
> TBR=tommi@webrtc.org
>
> Committed: https://crrev.com/9d34148714773339a4e8396bd28aceb571554d36
> Cr-Commit-Position: refs/heads/master@{#13484}

TBR=tommi@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.webrtc.org/2150293002
Cr-Commit-Position: refs/heads/master@{#13486}

build_overrides/webrtc.gni

@@ -23,8 +23,5 @@ rtc_include_pulse_audio = true
 # standalone WebRTC.
 rtc_include_internal_audio_device = true
 
-# There's no point processing WebRTC standalone tests in a Chromium build,
-# so give Chromium the option of overriding it here.
-declare_args() {
-  rtc_include_tests = true
-}
+# There's no point processing WebRTC standalone tests in a Chromium build.
+rtc_include_tests = true

webrtc/build/webrtc.gni

@@ -62,6 +62,7 @@ declare_args() {
   # https://gcc.gnu.org/wiki/LinkTimeOptimization
   rtc_use_lto = false
 
+  rtc_include_tests = false
   rtc_restrict_logging = true
 
   # Enable libevent task queues on platforms that support it.