libgav1,generate_libgav1_src_gni: run gn format on .gni

Change-Id: Ic6e92db4c24b7b13804f84824b82c89f26ab5bde
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2128945Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#754791}

third_party/libgav1/generate_libgav1_src_gni.go

@@ -132,6 +132,12 @@ func main() {
 		v := m[k]
 		format(k, v, file)
 	}
+	file.Close()
+
+	gnCmd := exec.Command("gn", "format", gniFile)
+	if gnCmd.Run() != nil {
+		panic(fmt.Sprintf("failed to execute gn format command: %v", err))
+	}
 
 	updateReadme()
 }