goma_ctl.py ensure_start in Soong UI.

As suggested in b/118390303#comment18, let me run
"goma_ctl.py ensure_start" in soong UI.

Bug: 118390303
Test: stop compiler_proxy, and execute following command:
Test: USE_GOMA=true lunch aosp_arm-eng
Test: and confirms compiler_proxy is running.
Change-Id: I859daae6ae2399c5b6bce2fafd874dfdb7c6aae0
Signed-off-by: Yoshisato Yanagisawa <yyanagisawa@google.com>
diff --git a/ui/build/build.go b/ui/build/build.go
index c902a0f..0ae06d6 100644
--- a/ui/build/build.go
+++ b/ui/build/build.go
@@ -156,6 +156,11 @@
 
 	SetupPath(ctx, config)
 
+	if config.StartGoma() {
+		// Ensure start Goma compiler_proxy
+		startGoma(ctx, config)
+	}
+
 	if what&BuildProductConfig != 0 {
 		// Run make for product config
 		runMakeProductConfig(ctx, config)