GN: Add modules_unittests

Changes:
* Enabled protobuf for iOS globally.
* Set WEBRTC_INCLUDE_INTERNAL_AUDIO_DEVICE on a global
scope similar to GYP since tests depend on it.
* Added missing rtc_libvpx_build_vp9 variable.
* Moved out audio_coding defines into .gni file to avoid code duplication
* Renamed files to avoid object naming conflicts that GN disallows:
  * webrtc/modules/audio_processing/{echo_cancellation_unittest.cc->echo_cancellation_bit_exact_unittest.cc}
  * webrtc/modules/video_coding/codecs/vp9/{screenshare_layers_unittest.cc->vp9_screenshare_layers_unittest.cc}

BUG=webrtc:5949
TESTED=Built and ran the tests on Mac. Also ran:
gn gen out/Default --args="rtc_enable_bwe_test_logging=true"
and verified that more objects are being built (1885 vs 1883)
when compiling modules_unittests.

NOTRY=True
NOPRESUBMIT=True

Review-Url: https://codereview.webrtc.org/2041233006
Cr-Commit-Position: refs/heads/master@{#13108}

webrtc/modules/video_coding/BUILD.gn

@@ -308,3 +308,33 @@ source_set("webrtc_vp9") {
     deps += [ rtc_libvpx_dir ]
   }
 }
+
+if (rtc_include_tests) {
+  source_set("video_codecs_test_framework") {
+    testonly = true
+    sources = [
+      "codecs/test/mock/mock_packet_manipulator.h",
+      "codecs/test/packet_manipulator.cc",
+      "codecs/test/packet_manipulator.h",
+      "codecs/test/predictive_packet_manipulator.cc",
+      "codecs/test/predictive_packet_manipulator.h",
+      "codecs/test/stats.cc",
+      "codecs/test/stats.h",
+      "codecs/test/videoprocessor.cc",
+      "codecs/test/videoprocessor.h",
+    ]
+
+    configs += [ "../..:common_config" ]
+    public_configs = [ "../..:common_inherited_config" ]
+
+    if (is_clang) {
+      # Suppress warnings from Chrome's Clang plugins.
+      # See http://code.google.com/p/webrtc/issues/detail?id=163 for details.
+      configs -= [ "//build/config/clang:find_bad_constructs" ]
+    }
+
+    deps = [
+      "../../test:test_support",
+    ]
+  }
+}