Add microfactory tracing

Test: m clean; load out/build.trace.gz in chrome://tracing
Test: m -j nothing; load again
Change-Id: I67d4e006a4bdde593b54c20d6d93a48278fe696d
diff --git a/ui/tracer/tracer.go b/ui/tracer/tracer.go
index f19ac18..8705040 100644
--- a/ui/tracer/tracer.go
+++ b/ui/tracer/tracer.go
@@ -45,6 +45,7 @@
 	End(thread Thread)
 	Complete(name string, thread Thread, begin, end uint64)
 
+	ImportMicrofactoryLog(filename string)
 	ImportNinjaLog(thread Thread, filename string, startOffset time.Time)
 
 	NewThread(name string) Thread