Add a unit test for file_tasks::FindFileBrowserHandlerTasks()

BUG=269929
TEST=none
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/26565012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228080 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228080