Add proguard pre-processing of google_play_services.

build/secondary/third_party/android_tools/BUILD.gn

Index: build/secondary/third_party/android_tools/BUILD.gn
diff --git a/build/secondary/third_party/android_tools/BUILD.gn b/build/secondary/third_party/android_tools/BUILD.gn
index dc934cb93e40b50637fdfd939f6726d272f02e3e..8c75044b7a8a4c6414232739659b7fa0fbd25b6d 100644
--- a/build/secondary/third_party/android_tools/BUILD.gn
+++ b/build/secondary/third_party/android_tools/BUILD.gn
@@ -88,7 +88,10 @@ android_resources("google_play_services_default_resources") {
 android_java_prebuilt("google_play_services_default_java") {
   deps = [
     ":android_support_v13_java",
+    ":android_support_v7_mediarouter_java",
     ":google_play_services_default_resources",
   ]
+  proguard_preprocess = true
+  proguard_config = "//third_party/android_tools/proguard.flags"
   jar_path = "$android_sdk_root/extras/google/google_play_services/libproject/google-play-services_lib/libs/google-play-services.jar"
 }