Improve a11y for diff viewing

Adds an aria-label to added and removed rows marking the row as "added"
or "removed" for screen readers. As part of this change, the <tr>
elements that make up the diff are now focusable.

Test plan:
- Somewhat included as a part of testing this functionality is the use
of WCT's a11ySuite function[1]. This will be introduced in a later
change.

[1] https://goo.gl/P15eE6

Bug: Issue 6435
Change-Id: I676fc171d404ca6e8e9276965192b1452595fa61
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 d101fea..a61b9ca 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -77,6 +77,9 @@
         font-family: var(--font-family);
         font-style: italic;
       }
+      .diff-row {
+        outline: none;
+      }
       .diff-row.target-row.target-side-left .lineNum.left,
       .diff-row.target-row.target-side-right .lineNum.right,
       .diff-row.target-row.unified .lineNum {
@@ -210,7 +213,10 @@
               is-image-diff="[[isImageDiff]]"
               base-image="[[_baseImage]]"
               revision-image="[[_revisionImage]]">
-            <table id="diffTable" class$="[[_diffTableClass]]"></table>
+            <table
+                id="diffTable"
+                class$="[[_diffTableClass]]"
+                role="presentation"></table>
           </gr-diff-builder>
         </gr-diff-highlight>
       </gr-diff-selection>