Files app: Enable muted autoplay for previewing video files.

Due to a Chrome policy change, autoplaying videos does not work in quick view.
Instead, we introduce muted autoplay to help users preview video files.

Bug: 804273
Test: Manually tested.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I07768b04ff3994f53811d22fd33613291dd5b896
Reviewed-on: https://chromium-review.googlesource.com/920049
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#536980}(cherry picked from commit 33e085fe31cc14110fc52092a35ee84c412b6bbb)
Reviewed-on: https://chromium-review.googlesource.com/928062
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/branch-heads/3325@{#528}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
diff --git a/ui/file_manager/file_manager/foreground/elements/files_safe_video_webview_content.html b/ui/file_manager/file_manager/foreground/elements/files_safe_video_webview_content.html
index 6c8813f..25f8e47 100644
--- a/ui/file_manager/file_manager/foreground/elements/files_safe_video_webview_content.html
+++ b/ui/file_manager/file_manager/foreground/elements/files_safe_video_webview_content.html
@@ -9,7 +9,7 @@
     <link rel="stylesheet" type="text/css" href="files_safe_video_webview_content.css">
   </head>
   <body>
-    <video hidden id="content" autoplay controls controlsList="nodownload">
+    <video hidden id="content" muted autoplay controls controlsList="nodownload">
     </video>
     <script src="files_safe_media_webview_content.js"></script>
   </body>