Reland of Mojo: Eliminate duplicate C API symbols

mojo/public/c/system/BUILD.gn

 # Copyright 2014 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.
 
-# Depend on this target to use the types etc defined in the system without
-# linking against a specific implementation of the system. To link against a
-# particular implementation, use the :for_component or
-# :for_shared_library targets, depending on the type of target you are.
-source_set("system") {
+component("system") {
   sources = [
     "buffer.h",
     "core.h",
     "data_pipe.h",
     "functions.h",
     "macros.h",
     "message_pipe.h",
     "platform_handle.h",
     "system_export.h",
+    "thunks.cc",
+    "thunks.h",
     "types.h",
     "wait_set.h",
   ]
+
+  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
 }
 
-# In an is_component_build build, everything can link against //mojo/edk/system
-# because it is built as a shared library. However, in a static build,
-# //mojo/edk/system is linked into an executable (e.g., mojo_shell), and must be
-# injected into other shared libraries (i.e., Mojo Apps) that need the mojo
-# system API.
-#
-# For component targets, add //mojo/public/c/system:for_component to your deps
-# section.
-#
-# For shared_library targets (e.g., a Mojo App), add
-# //mojo/public/c/system:for_shared_library to your deps
+# This should ONLY be depended upon directly by shared_library targets which
+# need to export the MojoSetSystemThunks symbol, like targets generated by the
+# mojo_native_application template in //mojo/public/mojo_application.gni.
+source_set("set_thunks_for_app") {
+  sources = [
+    "set_thunks_for_app.cc",
+  ]
+
+  public_deps = [
+    ":system",
+  ]
+}
+
+# TODO(rockot): Delete these deprecated aliases.
 
 group("for_shared_library") {
   public_deps = [
     ":system",
   ]
-  if (is_component_build) {
-    deps = [
-      "//mojo/edk/system",
-    ]
-  } else {
-    deps = [
-      "../../platform/native:system",
-    ]
-  }
 }
 
 group("for_component") {
   public_deps = [
     ":system",
   ]
-  if (is_component_build) {
-    deps = [
-      "//mojo/edk/system",
-    ]
-  }
 }