[NTP] Correct width and height of outline on selected tile

The width and height of the selected outline should be equal to
the dimensions of the parent subtract two times the pixel width
of the outline.

Bug: 899683
Change-Id: I9c2c8fc669651fe713e1aa431438c3fe02881543
Reviewed-on: https://chromium-review.googlesource.com/c/1335780Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608150}

chrome/browser/resources/local_ntp/custom_backgrounds.css

@@ -485,9 +485,10 @@ html[dir=rtl] .bg-sel-tile-bg {
 .selected-border {
   border: 2px solid rgba(26, 115, 232, .4);
   border-radius: 4px;
-  height: 113px;
+  box-sizing: border-box;
+  height: 100%;
   position: relative;
-  width: 152px;
+  width: 100%;
 }
 
 .selected-circle {