Adding in objc vars for WebRTC GN config.

Appears to be needed after https://codereview.chromium.org/1941053002.

R=ivoc@webrtc.org
TBR=kjellander@webrtc.org

Review URL: https://codereview.webrtc.org/1941413002 .

Cr-Commit-Position: refs/heads/master@{#12609}

build_overrides/gtest.gni

@@ -11,3 +11,9 @@ gtest_include_multiprocess = true
 
 # Include support for platform-specific operations across unit tests.
 gtest_include_platform_test = true
+
+# Exclude support for testing Objective C code on OS X and iOS.
+gtest_include_objc_support = true
+
+# Exclude support for flushing coverage files on iOS.
+gtest_include_ios_coverage = true