GN: Add default target

This reduces the number of objects when not specifying a
build target during compile. This is especially significant for Android
where the number of objects decreases from 3322 to 1761.

BUG=libyuv:644
R=fbarchard@google.com

Review URL: https://codereview.chromium.org/2395743002 .

BUILD.gn

@@ -19,6 +19,23 @@ config("libyuv_config") {
   }
 }
 
+# This target is built when no specific target is specified on the command line.
+group("default") {
+  testonly = true
+  deps = [
+    ":libyuv",
+  ]
+  if (libyuv_include_tests) {
+    deps += [
+      ":compare",
+      ":convert",
+      ":cpuid",
+      ":libyuv_unittest",
+      ":psnr",
+    ]
+  }
+}
+
 static_library("libyuv") {
   sources = [
     # Headers