Add restrictions for min and max width of the left nav in Files.app to avoid breaking layout.

Previously, it was easy to resize the nav, so the file list would disappear. We shouldn't let users break the ui.

TEST=Follow the bug's repro steps.
BUG=175958


Review URL: https://chromiumcodereview.appspot.com/12217164

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182196 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -169,6 +169,8 @@ body.loaded {
   -webkit-box-flex: 0;
   background-color: #e6e6e6;
   display: -webkit-box;
+  max-width: 50%;
+  min-width: 45px;
   overflow: hidden;
   position: relative;
   width: 168px;