Merge 199198 "CrOS Files.app: Prevent the middle panel from disa..."

> CrOS Files.app: Prevent the middle panel from disappearing.
> 
> r198571 causes the regression described in crbug/239152. This patch reverts a part of the patch to fix the issue.
> 
> BUG=239152
> TEST=manual
> NOTRY=True
> 
> Review URL: https://chromiumcodereview.appspot.com/14726003

TBR=yoshiki@chromium.org

Review URL: https://codereview.chromium.org/15112002

git-svn-id: svn://svn.chromium.org/chrome/branches/1500/src@199636 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/file_manager/css/file_manager.css

@@ -582,6 +582,7 @@ body[new-ui] .dialog-body {
 }
 
 body[new-ui] .dialog-middlebar-contents {
+  display: -webkit-box;
   max-width: 50%;
   min-width: 45px;
   position: relative;