Add button to settings page that leads to chrome://devices

Add button in the "cloud print" section that leads to chrome://devices, letting
users find local printers using DNS-SD. This CL goes hand in hand with
https://codereview.chromium.org/23537059/ to remove chrome://devices from the
uber page sidebar.

BUG=286157

Review URL: https://chromiumcodereview.appspot.com/23549037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224551 0039d316-1c4b-4281-b951-d872f2087c98
Fix crash that occurs when page is reloaded while "Email confirmation" dialog is displayed.

When a navigation occurs in a tab that shows a tab modal confirm dialog,
the OnClosed() method of this dialog is invoked. No new navigation should
happen at this time. Otherwise, the 2 calls to WebContentsImpl::NavigateToPendingEntry
(1 for the navigation that closed the dialog and 1 for the navigation that
occured from the OnClosed() callback) are nested and a crash occurs.

StartExplicitSync() (one_click_signin_helper.cc) now checks whether a
navigation is in progress before making a redirection to the NTP/Apps
page. No redirection is made if a navigation is in progress.

TEST=
1.Launch chrome and sign in to chrome, go to settings and disconnect account
2click on sign into chrome, sign out and sign in as different user, sign in with different user id
3.Blank page with confirmation box will appear, bookmark this page and drag it from bookmarks bar to omnibox
Expected result: no crash
BUG=293261

Review URL: https://chromiumcodereview.appspot.com/23736010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224552 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224552