QuickView: Improve security by rendering images inside webview.

ui/file_manager/file_manager/foreground/css/file_manager.css

Index: ui/file_manager/file_manager/foreground/css/file_manager.css
diff --git a/ui/file_manager/file_manager/foreground/css/file_manager.css b/ui/file_manager/file_manager/foreground/css/file_manager.css
index cbe6d0ec6c48dbc895d94eeaea99af427b17b2f2..8f51b53ad854619cae784a330b723623290e75d3 100644
--- a/ui/file_manager/file_manager/foreground/css/file_manager.css
+++ b/ui/file_manager/file_manager/foreground/css/file_manager.css
@@ -2421,6 +2421,10 @@ files-toggle-ripple {
   top: 0;
 }
 
+#quick-view {
+  z-index: 505; /* Must be above the scroll bar (500). */
+}
+
 /* Preventing FOUC */
 paper-input-container:unresolved {
   display: none;