Milo: Show accurate pending build numbers for builder view.

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 a88ac3897ae834009d5c82bd0d13bdff802b238c..9dd67d0091e7dd71e161493b5c6ca0fb748eb2c5 100644
--- a/milo/api/resp/builder.go
+++ b/milo/api/resp/builder.go
@@ -61,9 +61,12 @@ type Builder struct {
 	// Warning text, if any.
 	Warning string
 
-	CurrentBuilds  []*BuildSummary
-	PendingBuilds  []*BuildSummary
-	FinishedBuilds []*BuildSummary
+	CurrentBuilds []*BuildSummary
+	PendingBuilds []*BuildSummary
+	// PendingBuildNum is the number of pending builds, since the slice above
+	// may be a snapshot instead of the full set.
+	PendingBuildNum int
+	FinishedBuilds  []*BuildSummary
 
 	// MachinePool is primarily used by buildbot builders to list the set of
 	// machines that can run in a builder.  It has no meaning in buildbucket or dm