Revert of Mojo: Eliminate duplicate C API symbols

mojo/public/c/system/BUILD.gn

Index: mojo/public/c/system/BUILD.gn
diff --git a/mojo/public/c/system/BUILD.gn b/mojo/public/c/system/BUILD.gn
index 5d7085decdd849dc012bee707e438dc6a60fd243..c876027690c64e8788c44f9c160cbe4864e1d3a8 100644
--- a/mojo/public/c/system/BUILD.gn
+++ b/mojo/public/c/system/BUILD.gn
@@ -2,7 +2,11 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-component("system") {
+# 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") {
   sources = [
     "buffer.h",
     "core.h",
@@ -12,38 +16,45 @@
     "message_pipe.h",
     "platform_handle.h",
     "system_export.h",
-    "thunks.cc",
-    "thunks.h",
     "types.h",
     "wait_set.h",
   ]
-
-  defines = [ "MOJO_SYSTEM_IMPLEMENTATION" ]
 }
 
-# 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.
+# 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
 
 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",
+    ]
+  }
 }