[webui][ntp] Prevent history entry when changing background image

The history entry appeared because we set the background image URL by
setting src of the background image iframe. This CL changes this to
contentWindow.location.replace, which does not add a history entry.

Fixed: 1114089
Change-Id: I2038de67b7fab71fc732152357a43c3fd885f7d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347317
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797130}

chrome/browser/resources/new_tab_page/background_manager.js

@@ -112,7 +112,9 @@ export class BackgroundManager {
       this.loadTimeResolver_.reject();
       this.loadTimeResolver_ = null;
     }
-    this.backgroundImage_.src = url.href;
+    // We use |contentWindow.location.replace| because reloading the iframe by
+    // setting its |src| adds a history entry.
+    this.backgroundImage_.contentWindow.location.replace(url.href);
   }
 
   /**

chrome/test/data/webui/new_tab_page/background_manager_test.js

@@ -35,6 +35,13 @@ suite('NewTabPageBackgroundManagerTest', () => {
 
     backgroundImage = document.createElement('div');
     backgroundImage.id = 'backgroundImage';
+    backgroundImage.contentWindow = {
+      location: {
+        replace: url => {
+          backgroundImage.url = url;
+        }
+      }
+    };
     document.body.appendChild(backgroundImage);
 
     backgroundManager = new BackgroundManager();
@@ -64,7 +71,7 @@ suite('NewTabPageBackgroundManagerTest', () => {
     backgroundManager.setBackgroundImage({url: {url: 'https://example.com'}});
 
     // Assert.
-    assertEquals(wrapImageUrl('https://example.com'), backgroundImage.src);
+    assertEquals(wrapImageUrl('https://example.com'), backgroundImage.url);
   });
 
   test('setting custom style updates src', () => {
@@ -86,7 +93,7 @@ suite('NewTabPageBackgroundManagerTest', () => {
         `url2x=${encodeURIComponent('https://example2x.com')}&` +
         'size=cover&repeatX=no-repeat&repeatY=repeat&positionX=left&' +
         'positionY=top';
-    assertEquals(expected, backgroundImage.src);
+    assertEquals(expected, backgroundImage.url);
   });
 
   test.skip('receiving load time resolves promise', async () => {