DevTools: Add details text for page performance metrics.

Drive-by: Fix an exception when a selected node is panned offscreen.

BUG=901165

Change-Id: I26ae15f3506f89a93e3f91f5aa6715c3f0033b1a
Reviewed-on: https://chromium-review.googlesource.com/c/1352627
Commit-Queue: Paul Irish <paulirish@chromium.org>
Reviewed-by: Paul Irish <paulirish@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612048}

third_party/blink/renderer/devtools/front_end/perf_ui/FlameChart.js

@@ -1610,11 +1610,17 @@ PerfUI.FlameChart = class extends UI.VBox {
     const timelineData = this._timelineData();
     const startTime = timelineData.entryStartTimes[entryIndex];
     const duration = timelineData.entryTotalTimes[entryIndex];
-    let barX, barWidth;
+    let barX = 0;
+    let barWidth = 0;
+    let visible = true;
     if (Number.isNaN(duration)) {
       const position = this._markerPositions.get(entryIndex);
-      barX = position.x;
-      barWidth = position.width;
+      if (position) {
+        barX = position.x;
+        barWidth = position.width;
+      } else {
+        visible = false;
+      }
     } else {
       barX = this._chartViewport.timeToPosition(startTime);
       barWidth = duration * this._chartViewport.timeToPixel();
@@ -1632,7 +1638,7 @@ PerfUI.FlameChart = class extends UI.VBox {
     style.top = barY + 'px';
     style.width = barWidth + 'px';
     style.height = barHeight - 1 + 'px';
-    element.classList.remove('hidden');
+    element.classList.toggle('hidden', !visible);
     this._viewportElement.appendChild(element);
   }