We should use webview to avoid security risk.

I found we can support text preview just using webview.
https://drive.google.com/open?id=0B7EYjkGAjRAhU3g5a244TXR3b0U
Does it look good?

+sgabriel@ for UI review.

A few questions:

Does it carry formatting?
Does it cary typography? Was you document looking like this, even not in preview mode?
What happens for multi-paged documents?

Thanks @oka!

@sgabriel - I think that since these are plain text files, formatting, typography, and paging don't apply in these cases?

Right. It doesn't carry formatting etc., because it's rendered in a separate Chrome process. The behaviour is the same as when you drag & drop a text file into a Chrome tab.

Got it. Looks good to me then.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/885a00341b184c0daed52a7a7fb234f6f14638f6

commit 885a00341b184c0daed52a7a7fb234f6f14638f6
Author: oka <oka@chromium.org>
Date: Thu Dec 15 04:38:48 2016

Quick View: Support pdf and text preview.

This CL allows user to open files which are browsable on Chrome
on Quick View. Browsable files include pdf and text.
Screencast: https://bugs.chromium.org/p/chromium/issues/detail?id=640696#c11

BUG= 642713 , 640696 
TEST=manually tested the following:
- For pdf and text, preview with 24px of vertical margin appears.
- For image, video and audio, preview with 15% of vertical margin
appears as before.
- For .docx, 'no preview available' is shown as expected, though the
file type is 'document' which is the same as pdf.

CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2540863004
Cr-Commit-Position: refs/heads/master@{#438744}

[modify] https://crrev.com/885a00341b184c0daed52a7a7fb234f6f14638f6/ui/file_manager/file_manager/foreground/elements/files_quick_view.css
[modify] https://crrev.com/885a00341b184c0daed52a7a7fb234f6f14638f6/ui/file_manager/file_manager/foreground/elements/files_quick_view.html
[modify] https://crrev.com/885a00341b184c0daed52a7a7fb234f6f14638f6/ui/file_manager/file_manager/foreground/elements/files_quick_view.js
[modify] https://crrev.com/885a00341b184c0daed52a7a7fb234f6f14638f6/ui/file_manager/file_manager/foreground/js/quick_view_controller.js

58.0.3029.19/9334.11.0