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Both task_manager_view and task_manager_cocoa define ShowTaskManager(Browser*).
http://crrev.com/987483003 mistakenly causes both to get linked
when building for Mac. This patch undoes that by moving the task
manager files to Cocoa and Views sources. They are no longer inside
enabled_task_manager condition check, but they are still excluded
from Android this way.

BUG=425229

Review URL: https://codereview.chromium.org/1003323005

Cr-Commit-Position: refs/heads/master@{#322233}