ui: Avoid moving initial visible window start time past its end

- As of aosp/2649039, upon loading a trace, we move the start of the
  initial visible window to the first ftrace event.
- In some traces, the first ftrace event is after the end of the initial
  visible window calculated from the tracing metadata.
- On loading these traces, the UI would immediately crash as time spans
  of negative duration are not allowed.
- This CL limits how far back we can move the start of the visible
  window, avoiding negative time spans.

Bug: 325015524
Change-Id: I087318ed3fba51353d47afae106d7c9c51a77f00

ui/src/controller/trace_controller.ts

@@ -1159,9 +1159,11 @@ async function computeVisibleTime(
     visibleStart = Time.max(visibleStart, reliableRangeStart);
   }
 
+  // Move start of visible window to the first ftrace event
   const ftraceBounds = await computeFtraceBounds(engine);
   if (ftraceBounds !== null) {
-    visibleStart = ftraceBounds.start;
+    // Avoid moving start of visible window past its end!
+    visibleStart = Time.min(ftraceBounds.start, visibleEnd);
   }
   return HighPrecisionTimeSpan.fromTime(visibleStart, visibleEnd);
 }