Bug 1382486 - Mark reftest 1375315-12.html as random on webrender for frequent intermittent failures; r=kats

layout/reftests/bugs/reftest.list

@@ -2026,7 +2026,7 @@ fails-if(!stylo||styloVsGecko) == 1365162-1.html 1365162-1-ref.html
 == 1375315-9.html 1375315-9-ref.html
 == 1375315-10.html 1375315-10-ref.html
 random-if(webrender) == 1375315-11.html 1375315-11-ref.html
-== 1375315-12.html 1375315-12-ref.html
+random-if(webrender) == 1375315-12.html 1375315-12-ref.html
 == 1374062.html 1374062-ref.html
 == 1375513.html 1375513-ref.html
 == 1375674.html 1375674-ref.html