Bug 1419096 - Explicitly declare 0% value in @keyframes animation because start/end values are needed to calculate cumulative change hints. r=dao

See https://bugzilla.mozilla.org/show_bug.cgi?id=1419079#c22 for more information.

MozReview-Commit-ID: IZjtRfuUbN8

--HG--
extra : rebase_source : 4484de8f31718ae37ca938bd1c7b40bda5f6c1e7

browser/themes/shared/tabs.inc.css

@@ -186,10 +186,12 @@ tabbrowser {
 }
 
 @keyframes tab-throbber-animation {
+  0% { transform: translateX(0); }
   100% { transform: translateX(-100%); }
 }
 
 @keyframes tab-throbber-animation-rtl {
+  0% { transform: translateX(0); }
   100% { transform: translateX(100%); }
 }