Use chrome.tabs.create API for opening help page from Files.app.

When the help pages in chrome-extension:// URL, it cannot be opened from
<a href="..."> links. Along the way, strictified the condition to use
the offline help URL.

BUG=333389

Review URL: https://codereview.chromium.org/143883002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246573 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246573