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

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

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
index 99da821..e423e45 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
@@ -131,6 +131,16 @@
           'moveToPreviousCommentThread');
       MockInteractions.pressAndReleaseKeyOn(element, 80, ['shift']);  // 'P'
       assert(scrollStub.calledOnce);
+
+      var computeContainerClassStub = sandbox.stub(element.$.diff,
+          '_computeContainerClass');
+      MockInteractions.pressAndReleaseKeyOn(element, 74);  // 'j'
+      assert(computeContainerClassStub.lastCall.calledWithExactly(
+          false, 'SIDE_BY_SIDE', true));
+
+      MockInteractions.pressAndReleaseKeyOn(element, 27);  // 'escape'
+      assert(computeContainerClassStub.lastCall.calledWithExactly(
+          false, 'SIDE_BY_SIDE', false));
     });
 
     test('saving diff preferences', function() {