[quickview] Update QuickView test for transparent <webview> background

Fix test after CL:1933859, now the background color is transparent.

No-try: true
No-presubmit: true
Bug: 742944
Change-Id: Ib9af5c847f5453394d8f2062f3bd97f9d2a1c901
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1933864Reviewed-by: Austin Tankiang <austinct@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718538}

ui/file_manager/integration_tests/file_manager/quick_view.js

@@ -461,7 +461,7 @@
 
   /**
    * Tests opening Quick View on a text document to verify that the background
-   * color of the <webview> root (html) element is solid white.
+   * color of the <webview> root (html) element.
    */
   testcase.openQuickViewBackgroundColorText = async () => {
     const caller = getCaller();
@@ -501,8 +501,8 @@
     const backgroundColor = await remoteCall.callRemoteTestUtil(
         'deepExecuteScriptInWebView', appId, [webView, getBackgroundStyle]);
 
-    // Check: the <webview> root backgroundColor should be solid white.
-    chrome.test.assertEq('rgb(255, 255, 255)', backgroundColor[0]);
+    // Check: the <webview> root backgroundColor should be transparent.
+    chrome.test.assertEq('rgba(0, 0, 0, 0)', backgroundColor[0]);
   };
 
   /**