Add //mojo/edk/platform and move platform_task_runners.h there.

mojo/edk/platform/BUILD.gn

Index: mojo/edk/platform/BUILD.gn
diff --git a/mojo/edk/base_edk/BUILD.gn b/mojo/edk/platform/BUILD.gn
similarity index 55%
copy from mojo/edk/base_edk/BUILD.gn
copy to mojo/edk/platform/BUILD.gn
index b58bfe4c8deeb737c2adc7a8dd93a40c12c7770f..2d69e67c3a44074789fc8a824e1a92c1c6480227 100644
--- a/mojo/edk/base_edk/BUILD.gn
+++ b/mojo/edk/platform/BUILD.gn
@@ -4,15 +4,17 @@
 
 import("../mojo_edk.gni")
 
-mojo_edk_source_set("base_edk") {
+mojo_edk_source_set("platform") {
   sources = [
-    "platform_task_runner_impl.cc",
-    "platform_task_runner_impl.h",
+    "task_runner.h",
   ]
 
+  mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
+
+  # TODO(vtl): Remove this dependency.
   deps = [
     "//base",
   ]
 
-  mojo_edk_public_deps = [ "mojo/edk/system" ]
+  mojo_edk_public_deps = [ "mojo/edk/util" ]
 }