Fix About link for RTL

I realized I used margin-right instead of the direction-neutral
-webkit-margin-end in the M40 fix, so the margin between
the About link and the search box will be too small in RTL.

This fix is already included in the M38 CL.

BUG=421236
R=stevenjb@chromium.org

Review URL: https://codereview.chromium.org/655963005

Cr-Commit-Position: refs/heads/master@{#300517}

chrome/browser/resources/options/browser_options.css

@@ -474,7 +474,7 @@ footer {
 }
 
 #about-button {
-  margin-right: 30px;
+  -webkit-margin-end: 30px;
 }
 
 /* An input that has no function except to take up the same amount of space as