Fix the target row highlight on Firefox

Previiously, when pressing j/k on a change view and a horizontal line
appeared, in firefox, the rows jumped around a bit, as the bottom
border added an additional pixel of height.

Instead, this change uses a box shadow to simulate a bottom border.
The box shadow approach did not work on the table row, but instead had
to be applied to the table cell itself (in the target row)

Bug: Issue 6079
Change-Id: I7b80ef119f5ef0c8ad6cad51858edf2ac9c325da

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 6aae8d1..ebbc7e1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -158,8 +158,8 @@
       .contextControl td:not(.lineNum) {
         text-align: center;
       }
-      .displayLine .diff-row.target-row {
-        border-bottom: 1px solid #bbb;
+      .displayLine .diff-row.target-row td {
+        box-shadow: inset 0 -1px #bbb;
       }
       .br:after {
         /* Line feed */