[android] Write a runner script to the top-level output dir for gtests.

Android GN builds have been unable to build raw "unit_tests" as a target,
instead relying on unit_tests_apk or chrome/test:unit_tests. Linux builds,
in contrast, can do so without issue. I had thought that the issue was
that a "unit_tests" gn target in another directory was preventing gn
from creating a top-level alias. While this may be true, it's not the
reason for the discrepancy between Android and Linux builds. Instead,
the difference is that Linux builds create a unit_tests executable in
the top-level output directory, while Android builds create an APK and
a wrapper script in child directories.

This CL writes an additional wrapper script for gtest targets on Android
in the top-level directory that matches the suite's output_name (usually
its target name), allowing us to successfully build raw "unit_tests" on
Android. As a bonus, we can now run

  $ out/Release/unit_tests

(etc) on Android.

BUG=716117

Review-Url: https://codereview.chromium.org/2851523002
Cr-Commit-Position: refs/heads/master@{#469485}

build/config/android/internal_rules.gni

@@ -615,7 +615,12 @@ template("test_runner_script") {
       test_runner_args += [ "--tool=asan" ]
     }
 
-    generated_script = "$root_build_dir/bin/run_${_test_name}"
+    if (defined(invoker.generated_script)) {
+      assert(_test_name != "" || true)  # Mark _test_name as used.
+      generated_script = invoker.generated_script
+    } else {
+      generated_script = "$root_build_dir/bin/run_${_test_name}"
+    }
     outputs = [
       generated_script,
     ]

testing/test.gni

@@ -190,22 +190,35 @@ template("test") {
       }
     }
 
-    group(target_name) {
-      testonly = true
-      if (_incremental_apk_only) {
-        deps = [
-          ":${target_name}_incremental",
-        ]
-      } else {
-        deps = [
+    test_runner_script(target_name) {
+      forward_variables_from(invoker,
+                             _wrapper_script_vars + [
+                                   "data",
+                                   "data_deps",
+                                   "deps",
+                                   "public_deps",
+                                 ])
+
+      if (_use_raw_android_executable) {
+        executable_dist_dir = "$root_out_dir/$_dist_target"
+        deps += [
+          ":$_dist_target",
           ":$_test_runner_target",
         ]
-        if (_use_raw_android_executable) {
-          deps += [ ":$_dist_target" ]
+      } else {
+        apk_target = ":$_apk_target"
+        deps += [ ":$_apk_target" ]
+        if (_incremental_apk_only) {
+          deps += [ ":${target_name}_incremental" ]
         } else {
-          deps += [ ":$_apk_target" ]
+          deps += [ ":$_test_runner_target" ]
         }
       }
+      generated_script = "$root_build_dir/$_output_name"
+      incremental_install = _incremental_apk_only
+      test_name = _output_name
+      test_suite = _output_name
+      test_type = "gtest"
     }
 
     # TODO(GYP_GONE): Delete this after we've converted everything to GN.