DevTools: Fix exception when User Timings track is selected on the flamechart.

BUG=820250

Change-Id: I0bda360c1bf70887a56937ce0fc42f05c9d37083
Reviewed-on: https://chromium-review.googlesource.com/990193Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547489}

third_party/WebKit/Source/devtools/front_end/timeline/TimelineUIUtils.js

@@ -1061,6 +1061,8 @@ Timeline.TimelineUIUtils = class {
    * @return {!Object<string, number>}
    */
   static statsForTimeRange(events, startTime, endTime) {
+    if (!events.length)
+      return {'idle': endTime - startTime};
     const symbol = Timeline.TimelineUIUtils._categoryBreakdownCacheSymbol;
     Timeline.TimelineUIUtils._buildRangeStatsCacheIfNeeded(events);