Update CSS for supervised user block interstitial to be more friendly to narrow screens.

chrome/browser/resources/supervised_user_block_interstitial.css

Index: chrome/browser/resources/supervised_user_block_interstitial.css
diff --git a/chrome/browser/resources/supervised_user_block_interstitial.css b/chrome/browser/resources/supervised_user_block_interstitial.css
index 257924699a8fa37be34d74aaf1211f3c1a5e8080..8c3cae79717fe505455804c44680b535028c98b5 100644
--- a/chrome/browser/resources/supervised_user_block_interstitial.css
+++ b/chrome/browser/resources/supervised_user_block_interstitial.css
@@ -52,7 +52,7 @@ a {
 }
 
 #content-top {
-  margin: 20px 20px 20px 25px;
+  margin: 20px 25px;
 }
 
 button {
@@ -66,7 +66,7 @@ button {
   box-shadow: inset 0 1px 0 rgb(255, 255, 255);
   color: rgb(102, 102, 102);
   font-weight: bold;
-  margin: 0 5px;
+  margin: 5px 0;
   padding: 8px 13px;
   text-shadow: 0 1px 0 rgba(255, 255, 255, 0.8);
 }
@@ -80,5 +80,16 @@ button {
   box-shadow: inset 0 1px 0 rgb(95, 168, 247);
   color: rgb(255, 255, 255);
   text-shadow: 0 1px 0 rgba(0, 0, 0, 0.2);
-  width: 100px;
+}
+
+@media (min-width: 376px) {
+  #back-button {
+    width: 100px;
+  }
+}
+
+@media (max-width: 375px) {
+  button {
+    width: 100%;
+  }
 }