Add generate_jar_jni template

ui/snapshot/BUILD.gn

Index: ui/snapshot/BUILD.gn
diff --git a/ui/snapshot/BUILD.gn b/ui/snapshot/BUILD.gn
index 628812bde921b1131fdd977acb2d84c07db3445f..78a28237d0f6e79e7e6be2872cd5865cb81dd814 100644
--- a/ui/snapshot/BUILD.gn
+++ b/ui/snapshot/BUILD.gn
@@ -45,6 +45,10 @@ component("snapshot") {
       #"//ui/aura",  TODO(GYP)
       "//ui/compositor",
     ]
+  } else {
+    sources -= [

[cjhopman, 2014/06/04 23:10:12]

I don't know that this is how we actually want to filter aura stuff...

gyp uses a sources filter for it, but it didn't seem like we had any of those
for non-OS concepts (and I, personally, really don't like the magic that is
sources filters).

[tfarina, 2014/06/05 02:12:27]

From what I understood from Brett's reply, that is how we want to do. So we,
that looks right.

For reference, Brett's reply is here:
https://codereview.chromium.org/305993002/diff/160001/ui/base/BUILD.gn#oldcod...

[brettw, 2014/06/05 02:38:15]

Yeah, this is what I have been doing.

+      "snapshot_aura.cc"
+    ]
   }
 }