Migrate from string paths Entries in file_tasks.js.

This patch migrates from full paths as strings to Entries wherever possible in file_tasks.js.

TEST=Tested manually. Also, browser_tests.
BUG=323753

Review URL: https://codereview.chromium.org/91123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238656 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238656