truncate microsummary file names to 60 characters to prevent nsIFile.createUnique from failing due to too-long names

Patch by Joey Minta.
bug=342212
r=myk

browser/components/microsummaries/src/nsMicrosummaryService.js

@@ -1829,6 +1829,7 @@ function getLoadedMicrosummaryResource(uri) {
  */
 function sanitizeName(aName) {
   const chars = "-abcdefghijklmnopqrstuvwxyz0123456789";
+  const maxLength = 60;
 
   var name = aName.toLowerCase();
   name = name.replace(/ /g, "-");
@@ -1847,6 +1848,9 @@ function sanitizeName(aName) {
       name += chars.charAt(Math.round(Math.random() * (chars.length - 1)));
   }
 
+  if (name.length > maxLength)
+    name = name.substring(0, maxLength);
+
   return name;
 }