[NTP] Set min width for buttons in custom background dialog

Set min width for cancel and done button to make it to fix the extra
space shown on rtl mode when the "cancel" and "done" are too short in
another language.

Bug: 876251
Change-Id: I588ed62bf28249c2078edb10a0d29143041f3d92
Reviewed-on: https://chromium-review.googlesource.com/1184164
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585165}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -344,6 +344,7 @@ html[dir=rtl] .is-img-sel #bg-sel-title {
   font-weight: 500;
   height: 32px;
   line-height: normal;
+  min-width: 60px;
   padding: 0 16px;
   position: absolute;
   text-align: center;