Add tests for showing the thumbnail via the image loader.

This patch adds a simple test, which checks if the thumbnail with the preview of the selected image shows up. Since showing the thumbnail calls the image loader, this test verifies basic behaviour of the image loader.

TEST=browser_tests:*Thumbnails*FileManagerBrowserTest*
BUG=314915

Review URL: https://codereview.chromium.org/82583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236731 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236731