Add buck build soyc to output GWT split point report

The GWT compiler can output a report (or "story of your compile"),
describing the size of the JavaScript and which source classes
contributed to the overall download size. This is useful for
optimizing code splits in Dispatcher, and generally understanding
the impact of linking code together.

Only the "Split Point Report" is created.
The "Compiler Metrics" are not output.

The report is very slow to create, so it is not done by default.

Change-Id: I6ce86d65ee5f3e4bdba573d1f777ccc59767341d
diff --git a/.gitignore b/.gitignore
index b356144..c30cee6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 /.buckd
 /buck-cache
 /buck-out
+/extras
 /local.properties
 *.pyc
 /gwt-unitCache