Early out if contentviewcore native side is null.

BUG=b/10911057

Early out before calling native getNavigatonHistory as it may be
deleted before.

Review URL: https://codereview.chromium.org/25505005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226361 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226361
Migrate to Styx thumbnails in Files App.

Styx provides thumbnails using a persistent urls, in contrast to the previous urls which used to get stale with time. This patch moves to the persisten urls, to there is no need to handle stale urls anymore.

TEST=browser_tests
BUG=254025

Review URL: https://codereview.chromium.org/25458002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226362 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226362