Bug 1838398 - Add env variable to override output dir for external performance marker files. r=mstange

Currently these get output to the current working directory. On
Android, however, we cannot write to that directory, so this
environment variable can be used to set a directory that is writeable.

Differential Revision: https://phabricator.services.mozilla.com/D190289

dom/performance/Performance.cpp

@@ -573,6 +573,9 @@ DOMHighResTimeStamp Performance::ResolveStartTimeForMeasure(
 
 static std::string GetMarkerFilename() {
   std::stringstream s;
+  if (char* markerDir = getenv("MOZ_PERFORMANCE_MARKER_DIR")) {
+    s << markerDir << "/";
+  }
 #ifdef XP_WIN
   s << "marker-" << GetCurrentProcessId() << ".txt";
 #else