FilesApp add support for QuickView in test.html

Add support in chrome.fileManagerPrivate.getFileTasks to return
file task 'view-in-browser' for a single text file.  This is
required by QuickView to determine that a text file can be shown.

Change QuickView-related files to use iframe rather than webview.
Note that this change is only used for testing parts of the UI.
Production code is unchanged and still uses webview for security.

Put files_quick_view.html path in constants so that it can be
overriden in test to use version of file with iframe vs webview.


Bug: 813477
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I5b8666a646ce979a99f065aa44a5c802b5420a32
Reviewed-on: https://chromium-review.googlesource.com/968102
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Sasha Morrissey <sashab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549093}
6 files changed