[NTP] Prevent text highlighted when double click on custom background dialog

This commit fix the text highlighted problem both on cancel/done
button and the custom background dialog title.

Bug: 869086
Change-Id: I6e67f241409ed5498f703ec48966da0105ee5925
Reviewed-on: https://chromium-review.googlesource.com/1171792Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582725}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -240,6 +240,7 @@ html[dir=rtl] #bg-sel-tiles {
   height: 51px;
   line-height: 52px;
   text-align: center;
+  user-select: none;
 }
 
 #bg-sel-back {
@@ -330,6 +331,7 @@ html[dir=rtl] .is-img-sel #bg-sel-title {
   height: 64px;
   padding-left: 0;
   position: absolute;
+  user-select: none;
   width: 100%;
 }
 
@@ -377,7 +379,7 @@ html[dir=rtl] #bg-sel-footer-cancel {
 }
 
 #bg-sel-footer-cancel:hover {
-  background-color: rgb(154, 160, 166, 0.04);
+  background-color: rgba(154, 160, 166, 0.04);
 }
 
 #bg-sel-footer-cancel:active {