Bug 1302681 - Part 2: Fix about:telemetry for displaying dynamic events. r=chutten

Fixes that about:telemetry only shows events if there are any present in processes.parent.

For some reason switching the processes in the events section also triggers HistogramSection.render(), which breaks when there are no histograms recorded in the selected process.
This patch fixes it to make things work.

--HG--
extra : rebase_source : e7a4cd1d823571a060a4d6293800b0900a68ddd4

toolkit/content/aboutTelemetry.js

@@ -2093,7 +2093,8 @@ var HistogramSection = {
 
     if (hgramsProcess === "parent") {
       histograms = aPayload.histograms;
-    } else if ("processes" in aPayload && hgramsProcess in aPayload.processes) {
+    } else if ("processes" in aPayload && hgramsProcess in aPayload.processes &&
+               "histograms" in aPayload.processes[hgramsProcess]) {
       histograms = aPayload.processes[hgramsProcess].histograms;
     }
 
@@ -2126,7 +2127,8 @@ var KeyedHistogramSection = {
     let keyedHgramsProcess = keyedHgramsOption.getAttribute("value");
     if (keyedHgramsProcess === "parent") {
       keyedHistograms = aPayload.keyedHistograms;
-    } else if ("processes" in aPayload && keyedHgramsProcess in aPayload.processes) {
+    } else if ("processes" in aPayload && keyedHgramsProcess in aPayload.processes &&
+               "keyedHistograms" in aPayload.processes[keyedHgramsProcess]) {
       keyedHistograms = aPayload.processes[keyedHgramsProcess].keyedHistograms;
     }