GitLab

Make arc_file_tasks.cc use the IntentHandlerInfo struct's
string "action" field rather than the ActionType enum.
ActionTypes are still used when calling HandleUrlList.

BUG=647802
TEST=manual: still able to launch ARC apps for individual
     files or send multiple files from files app

Review-Url: https://codereview.chromium.org/2398063003
Cr-Commit-Position: refs/heads/master@{#423797}