PDF Viewer: Change document rectangle focus color.

Per UX suggestion, experimenting with a different color,
google-grey-600 instead of white, which unfortunately is not
noticeable at all when the document (usually white bg color),
is zoomed in.

Bug: 1069370
Change-Id: Ibde70bac01ce70dde3e56a5a6129d549e53baf9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343568
Commit-Queue: dpapad <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796079}

chrome/browser/resources/pdf/pdf_viewer.html

@@ -5,7 +5,7 @@
   }
 
   #content-focus-rectangle {
-    border: 2px solid white;
+    border: 2px solid var(--google-grey-600);
     border-radius: 2px;
     box-sizing: border-box;
     height: 100%;

chrome/browser/resources/pdf/pdf_viewer.js

@@ -14,6 +14,7 @@ import './elements/viewer-form-warning.js';
 // </if>
 import './pdf_viewer_shared_style.js';
 import 'chrome://resources/cr_elements/hidden_style_css.m.js';
+import 'chrome://resources/cr_elements/shared_vars_css.m.js';
 
 import {assert, assertNotReached} from 'chrome://resources/js/assert.m.js';
 import {loadTimeData} from 'chrome://resources/js/load_time_data.m.js';