Make line marker more distinguished

Previously, the line marker was only subtly visible by the highlighted
line number. This change adds a bottom border to the selected line if
the user is using keycodes (j, k, up, down) to more the cursor. When
the escape key is pressed, the distinguished line marker will dissapear.

Feature: Issue 4739
Change-Id: If8c751efc137ef87cfdad1c8bf7d905de1219107
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 0816501..8262658 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -150,6 +150,9 @@
       .contextControl td:not(.lineNum) {
         text-align: center;
       }
+      .displayLine .diff-row.target-row {
+        border-bottom: 1px solid #bbb;
+      }
       .br:after {
         /* Line feed */
         content: '\A';
@@ -164,7 +167,7 @@
       }
     </style>
     <style include="gr-theme-default"></style>
-    <div class$="[[_computeContainerClass(_loggedIn, viewMode)]]"
+    <div class$="[[_computeContainerClass(_loggedIn, viewMode, displayLine)]]"
         on-tap="_handleTap">
       <gr-diff-selection diff="[[_diff]]">
         <gr-diff-highlight