Ensure libmojo_core.so is built for Chrome OS builds

Makes the mojo_core shared library a data dependency of the chrome
binary target on Chrome OS to ensure that it gets built whenever chrome
gets built. A follow-up change to chromite will cause this to be
included in device deployments.

Also adds a linker script to ensure that symbol exports are limited to
MojoGetSystemThunks.

Bug: 822034
Change-Id: Iad03f28377ecc5094c2019b22c723d7705242230
Reviewed-on: https://chromium-review.googlesource.com/1024750Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552901}

chrome/BUILD.gn

@@ -298,6 +298,10 @@ if (!is_android && !is_mac) {
         }
       }
 
+      if (is_chromeos) {
+        data_deps += [ "//mojo/edk:mojo_core" ]
+      }
+
       # These files are used by the installer so we need a public dep.
       public_deps += [ ":packed_resources" ]
 

mojo/edk/BUILD.gn

@@ -220,9 +220,12 @@ if (is_chromeos || is_linux) {
       "//build/config:exe_and_shlib_deps",
       "//mojo/public/c/system:headers",
     ]
-    if (!is_component_build) {
-      public_configs = [ "//build/config/gcc:rpath_for_built_shared_libraries" ]
-    }
+    configs += [ ":export_only_thunks_api" ]
+  }
+
+  config("export_only_thunks_api") {
+    ldflags = [ "-Wl,--version-script=" +
+                rebase_path("//mojo/edk/export_only_thunks_api.lst") ]
   }
 
   test("mojo_core_unittests") {

mojo/edk/export_only_thunks_api.lst

+# Copyright 2018 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Linker script that ensures only MojoGetSystemThunks is exposed from the
+# mojo_core library.
+{
+  global:
+    MojoGetSystemThunks;
+  local:
+    *;
+};