Fix border colors for offline content preview on dino page #2

Incorrect color used in the previous attempt
(https://crrev.com/c/1364297).

TBR=chili@chromium.org

Bug: 852872
Change-Id: Ib1fd4d2b39528d8ce6a0e41b13ad155af9ab02ab
Reviewed-on: https://chromium-review.googlesource.com/c/1366779
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614826}

components/neterror/resources/neterror.css

@@ -569,7 +569,7 @@ div.offline-content-suggestion {
 }
 
 #offline-content-summary {
-  border: 1px solid rgb(223, 225, 229);
+  border: 1px solid var(--google-gray-300);
   border-radius: 12px;
   padding: 12px;
   text-align: center;
@@ -598,7 +598,7 @@ div.offline-content-suggestion {
 }
 
 .offline-content-summary-description {
-  border-top: 1px solid rgb(223, 225, 229);
+  border-top: 1px solid var(--google-gray-300);
   padding-top: 12px;
 }