Bug 1387969 - show scrollbar when onboarding tour description is longer;r=rexboy

MozReview-Commit-ID: 12uhcn9EWJZ

--HG--
extra : rebase_source : 4ce0e7c35ee6003a508e9aa008ef5441e6692d4a

browser/extensions/onboarding/content/onboarding.css

@@ -251,6 +251,8 @@
   line-height: 22px;
   padding-inline-start: 40px;
   padding-inline-end: 28px;
+  max-height: 360px;
+  overflow: auto;
 }
 
 .onboarding-tour-description > h1 {