[milo] Add BuildSummary and common models.

milo/api/resp/builder.go

Index: milo/api/resp/builder.go
diff --git a/milo/api/resp/builder.go b/milo/api/resp/builder.go
index ca1d46045a87c5cb4245602105589d9d6307f60c..59e26a1de555ff84617faf5315dafa26d6a7de8c 100644
--- a/milo/api/resp/builder.go
+++ b/milo/api/resp/builder.go
@@ -6,7 +6,11 @@
 
 package resp
 
-import "time"
+import (
+	"time"
+
+	"github.com/luci/luci-go/milo/appengine/common/model"
+)
 
 // Interval is a time interval which has a start, an end and a duration.
 type Interval struct {
@@ -23,7 +27,7 @@ type BuildSummary struct {
 	Link *Link
 
 	// Status of the build.
-	Status Status
+	Status model.Status
 
 	// Pending is time interval that this build was pending.
 	PendingTime Interval