Fix line numbers in shadow-dom

Another border-box issue, this time with .lineNum:before.

Bug: Issue 6372
Change-Id: Ic6521090775f2a990b4fa7b7f36c5989151ce2ac

polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html

diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index a61b9ca..84d9783 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -110,6 +110,7 @@
       }
       .contextLineNum:before,
       .lineNum:before {
+        box-sizing: border-box;
         display: inline-block;
         color: #666;
         content: attr(data-value);