GN: Refactor modules_unittests to eliminate package boundary violations.

webrtc/modules/audio_mixer/BUILD.gn

Index: webrtc/modules/audio_mixer/BUILD.gn
diff --git a/webrtc/modules/audio_mixer/BUILD.gn b/webrtc/modules/audio_mixer/BUILD.gn
index b4c900d1509a3068f2de2759f19ff1acefb6fa4c..25c782bc9b9664ad6393df15ce389b8912536bd5 100644
--- a/webrtc/modules/audio_mixer/BUILD.gn
+++ b/webrtc/modules/audio_mixer/BUILD.gn
@@ -58,3 +58,33 @@ rtc_static_library("audio_frame_manipulator") {
     "../../base:rtc_base_approved",
   ]
 }
+
+if (rtc_include_tests) {
+  rtc_source_set("audio_mixer_unittests") {
+    # When enabled, fails with error audio_mixer_impl_unittest.cc includes a
+    # private header. Fie default_output_rate_calculator.h is private to the
+    # target :audio_mixer_impl
+    check_includes = false
+    testonly = true
+    sources = [
+      "audio_frame_manipulator_unittest.cc",
+      "audio_mixer_impl_unittest.cc",
+    ]
+    deps = [
+      ":audio_frame_manipulator",
+      ":audio_mixer_impl",
+      "../../api:audio_mixer_api",
+      "../../base:rtc_base",
+      "../../base:rtc_base_approved",
+      "../../test:test_support",
+      "//testing/gmock",
+    ]
+
+    # TODO(jschuh): bugs.webrtc.org/1348: fix this warning.
+    configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
+    if (!build_with_chromium && is_clang) {
+      # Suppress warnings from the Chromium Clang plugin (bugs.webrtc.org/163).
+      suppressed_configs += [ "//build/config/clang:find_bad_constructs" ]
+    }
+  }
+}