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Further, and hopefully the last step for refactoring needed for the feature.
 1) We need mime types on the task running time as well. Added a cycle for it.
 2) Split the task enumeration function for the placeholder for ARC stuff and
   post processing part.
 3) For the efficiency and clarity for the split of 2), changed std::vector
   value copying to movement by std::unique_ptr.

BUG=607059
TEST=unit_tests --gtest_filter='*FileManagerFileTask*'
TEST=browser_tests --gtest_filter='FileManagerJsTest.FileTasks'

Review-Url: https://codereview.chromium.org/1923953005
Cr-Commit-Position: refs/heads/master@{#390326}