Add build tracing

This creates a rotating build.trace.gz in the out directory that can be
loaded with chrome://tracing. It'll include start and end timings for
make/soong/kati/ninja, and it will import and time-correct the ninja log
files.

Test: m -j; load out/build.trace.gz in chrome://tracing
Test: multiproduct_kati -keep; load out/multiproduct*/build.trace.gz
Change-Id: Ic060fa9515eb88d95dbe16712479dae9dffcf626
diff --git a/ui/build/kati.go b/ui/build/kati.go
index 6997fbe..423bcbc 100644
--- a/ui/build/kati.go
+++ b/ui/build/kati.go
@@ -57,6 +57,9 @@
 }
 
 func runKati(ctx Context, config Config) {
+	ctx.BeginTrace("kati")
+	defer ctx.EndTrace()
+
 	genKatiSuffix(ctx, config)
 
 	executable := "prebuilts/build-tools/" + config.HostPrebuiltTag() + "/bin/ckati"