Force the QuickView background white when displaying text documents

In issue 883117 an externally injected stylesheet changed scroll style
of QuickView text documents: scroll bars went missing.

Investigating more, I find the background color of QuickView <webview>
content is also not set, so it is currently possible for an externally
injected stlyesheet to override the background color of text documents
in QuickView by accident.

Prevent such accidents - set the <webview> content background color to
white and add a test to verify the background color.

Bug: 891150
Change-Id: Ic7881ae45b2f36ba94ed245bae3d5944cda0eb2e
Reviewed-on: https://chromium-review.googlesource.com/c/1260784Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596517}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -388,6 +388,7 @@ WRAPPED_INSTANTIATE_TEST_CASE_P(
                       TestCase("openQuickView").TabletMode(),
                       TestCase("openQuickViewScrollText"),
                       TestCase("openQuickViewScrollHtml"),
+                      TestCase("openQuickViewBackgroundColorText"),
                       TestCase("openQuickViewBackgroundColorHtml"),
                       TestCase("openQuickViewDrive"),
                       TestCase("openQuickViewDrive").EnableDriveFs(),

ui/file_manager/file_manager/foreground/elements/files_safe_text_webview_content.css

@@ -19,6 +19,7 @@ body {
 }
 
 html {
+  background: white;
   overflow: auto;
 }
 

ui/file_manager/integration_tests/file_manager/quick_view.js

@@ -343,6 +343,69 @@ testcase.openQuickViewScrollText = function() {
   ]);
 };
 
+/**
+ * Tests opening Quick View on a text document to verify that the background
+ * color of the <webview> root (html) element is solid white.
+ */
+testcase.openQuickViewBackgroundColorText = function() {
+  const caller = getCaller();
+  let appId;
+
+  /**
+   * The text <webview> resides in the #quick-view shadow DOM, as a child of
+   * the #dialog element.
+   */
+  const webView = ['#quick-view', '#dialog[open] webview.text-content'];
+
+  StepsRunner.run([
+    // Open Files app on Downloads containing ENTRIES.tallText.
+    function() {
+      setupAndWaitUntilReady(
+          null, RootPath.DOWNLOADS, this.next, [ENTRIES.tallText], []);
+    },
+    // Open the file in Quick View.
+    function(results) {
+      appId = results.windowId;
+      const openSteps = openQuickViewSteps(appId, ENTRIES.tallText.nameText);
+      StepsRunner.run(openSteps).then(this.next);
+    },
+    // Wait for the Quick View <webview> to load and display its content.
+    function() {
+      function checkWebViewTextLoaded(elements) {
+        let haveElements = Array.isArray(elements) && elements.length === 1;
+        if (haveElements)
+          haveElements = elements[0].styles.display.includes('block');
+        if (!haveElements || !elements[0].attributes.src)
+          return pending(caller, 'Waiting for <webview> to load.');
+        return;
+      }
+      repeatUntil(function() {
+        return remoteCall
+            .callRemoteTestUtil(
+                'deepQueryAllElements', appId, [webView, ['display']])
+            .then(checkWebViewTextLoaded);
+      }).then(this.next);
+    },
+    // Get the <webview> root (html) element backgroundColor style.
+    function() {
+      const getBackgroundStyle =
+          'window.getComputedStyle(document.documentElement).backgroundColor';
+      remoteCall
+        .callRemoteTestUtil(
+            'deepExecuteScriptInWebView', appId, [webView, getBackgroundStyle])
+        .then(this.next);
+    },
+    // Check: the <webview> root backgroundColor should be solid white.
+    function(backgroundColor) {
+      chrome.test.assertEq('rgb(255, 255, 255)', backgroundColor[0]);
+      this.next();
+    },
+    function(results) {
+      checkIfNoErrorsOccured(this.next);
+    },
+  ]);
+};
+
 /**
  * Tests opening Quick View and scrolling its <webview> which contains a tall
  * html document.