Expose ability to download network service impl in SDK.

build/module_args/mojo.gni

Index: build/module_args/mojo.gni
diff --git a/build/module_args/mojo.gni b/build/module_args/mojo.gni
index db961e48671d0adde470ecea7a4cba35986777ea..52923eaa9b2f7907cf8f709043a7bfba661bc89a 100644
--- a/build/module_args/mojo.gni
+++ b/build/module_args/mojo.gni
@@ -6,3 +6,7 @@
 # shell will be used only on platforms for which it is published (currently
 # Linux and Android).
 use_prebuilt_mojo_shell = false
+
+# To use the prebuilt network service, set this variable to true or simply omit

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

Should both comments go the other way, ie. "By default a prebuilt network
service is used. To use a locally built network service set this variable to
false." ?

[jamesr, 2015/02/10 22:45:21]

Yeah, I think the default should be prebuilt (here the mojo repo is the odd one
out)

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

Changed the comments. The code defaults to prebuilt if the variable isn't
defined. ppi@ offline commented that it's a little bizarre to have undefined
mean "true" instead of "false", and that it would be more logical to have a
variable "build_network_service_from_source" that is false if undefined. I'm
happy either way; if changing this var, I think it would make sense to make a
corresponding change to use_prebuilt_mojo_shell.

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

Yeah, flipping both flags around sounds good to me, so does doing this in a
follow-up.

+# it.
+use_prebuilt_network_service = true