[DevTools] Add support for nav times in Flame characters

Adds support for nav start times ahead of
https://crrev.com/c/devtools/devtools-frontend/+/2252159

Bug: 974550
Change-Id: I67af15f1f21af81454e7683541646b1b8d4c0171
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252140
Auto-Submit: Paul Lewis <aerotwist@chromium.org>
Reviewed-by: Paul Lewis <aerotwist@chromium.org>
Reviewed-by: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Tim van der Lippe <tvanderlippe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779867}

third_party/blink/web_tests/http/tests/devtools/components/flame-chart.js

@@ -67,6 +67,10 @@
 
     highlightEntry(entryIndex) {
     }
+
+    navStartTimes() {
+        return [];
+    }
   }
 
   var entries = [