Use GN's dependency info for native libraries in write_build_config.py

This is in place of readelf / write_ordered_libaries.py.

Main motivation is to be able to write .build_config files quickly, so
that they can be used for generating build.gradle files for Android
Studio without first needing to build a bunch of native libraries.

BUG=620034
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2082453003
Cr-Commit-Position: refs/heads/master@{#401398}

diff --git a/testing/test.gni b/testing/test.gni
index d7aa7cf9..b93ef87 100644
--- a/testing/test.gni
+++ b/testing/test.gni

@@ -187,14 +187,12 @@
       }
       unittest_apk(_apk_target) {
         forward_variables_from(invoker, _apk_specific_vars + [ "deps" ])
-        unittests_dep = ":$_library_target"
+        shared_library = ":$_library_target"
         apk_name = invoker.target_name
         if (defined(invoker.output_name)) {
           apk_name = invoker.output_name
-          unittests_binary = "lib${apk_name}.so"
           install_script_name = "install_${invoker.output_name}"
         }
-        deps += [ ":$_library_target" ]
 
         # TODO(agrieve): Remove this data_dep once bots don't build the _apk
         #     target (post-GYP).