Add GN target for jsr305.jar

This JAR is a dependency of Robolectric 3.4.2.

R=jbudorick@chromium.org

Review-Url: https://codereview.chromium.org/2994903002 .
diff --git a/BUILD.gn b/BUILD.gn
index eb1b297..ccd1133 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -7,4 +7,8 @@
 # GYP: //third_party/findbugs:format_string_jar
 java_prebuilt("format_string_java") {
   jar_path = "lib/jFormatString.jar"
-}
\ No newline at end of file
+}
+
+java_prebuilt("jsr305_java") {
+  jar_path = "lib/jsr305.jar"
+}