GN: Don't build tests by default.

BUG=webrtc:6412
R=kjellander@webrtc.org, charujain@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2486753002
Cr-Commit-Position: refs/heads/master@{#14979}

BUILD.gn

@@ -6,12 +6,18 @@
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
+import("webrtc/build/webrtc.gni")
+
 group("default") {
   testonly = true
   deps = [
     "//webrtc",
+  ]
+  if (rtc_include_tests) {
+    deps += [
       "//webrtc:webrtc_tests",
       "//webrtc/examples",
       "//webrtc/tools",
     ]
   }
+}