Added PlatformHandle thunks.

Added thunks for exchanging PlatformHandles (file descriptors) between mojo services as MojoHandles. Basically a port of https://codereview.chromium.org/1113393002 plus some testing code. This is intended to be used to allow cross-process sharing of GPU buffers

Change-Id: I5c77f0ed0381c0b03ce0280314f7fbef3551d895

BUG=
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1578423002 .
diff --git a/shell/BUILD.gn b/shell/BUILD.gn
index 3068cff..3732d0a 100644
--- a/shell/BUILD.gn
+++ b/shell/BUILD.gn
@@ -269,6 +269,7 @@
   sources = [
     "native_application_support.cc",
     "native_application_support.h",
+    "platform_handle_impl.cc",
   ]
 
   public_deps = [
@@ -280,6 +281,7 @@
     "//mojo/gles2:control_thunks",
     "//mojo/gles2:gles2",
     "//mojo/gles2:mgl",
+    "//mojo/public/platform/native:platform_handle_api",
   ]
 
   # This target has to include the public thunk headers, which generally