Expose ability to download network service impl in SDK.

mojo/public/mojo.gni

Index: mojo/public/mojo.gni
diff --git a/mojo/public/mojo.gni b/mojo/public/mojo.gni
index 9f1b86b7d203dab69125d1c54555de3066e831d0..073da012483a4481cc0db4fd9365933416e6be84 100644
--- a/mojo/public/mojo.gni
+++ b/mojo/public/mojo.gni
@@ -10,6 +10,14 @@ if (!defined(use_prebuilt_mojo_shell) || use_prebuilt_mojo_shell) {
   use_prebuilt_mojo_shell = is_linux || is_android
 }
 
+# If using the prebuilt network service, gate its usage by the platforms for
+# which it is published.
+if (!defined(use_prebuilt_network_service) || use_prebuilt_network_service) {
+  # The prebuilt Mojo shell packages the network service on Android.

[ppi, 2015/02/10 16:49:50]

nit: I think s/packages/contains/ would stress that the network binary is
actually within the shell. wdyt?

[blundell, 2015/02/11 21:06:17]

Done.

+  use_prebuilt_network_service =
+      is_linux || (is_android && !use_prebuilt_mojo_shell)

[ppi, 2015/02/10 16:49:50]

Is this part needed? On Android the choice to pull a prebuilt network service or
to build locally doesn't matter, and it doesn't matter symetrically for both
cases. Not sure why would we override the behavior of pulling down binaries
versus the local build one way or another.

Wdyt?

[blundell, 2015/02/11 21:06:17]

My thought is that it's weird and a little confusing to pull a
network_service.mojo artifact into the build dir that's never used on Android. I
agree that it wouldn't actually cause any problems.

[ppi, 2015/02/12 13:36:49]

Right, but isn't building it locally when it's never used less weird? I would
err on the side of simplicity and drop the android condition here. Wdyt?

[blundell, 2015/02/12 13:57:41]

Yes, you're right (I think you meant *more* weird). I hadn't thought about the
Chromium case, where are you actually *are* building the network service from
source if you're not using the prebuilt network service. In that case, it would
be totally unexpected for someone to set use_prebuilt_network_service to true
and then have the network service be built from source on Android anyway

+}
+
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location