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There were circular deps between file_manager_commands and file_manager.
This change removes dependency from file_manager_commands to
file_manager by extracting an interface from FileManager needed for
file_manager_commands to compile.
Also inlined some simple methods in FileManager, which are used
only from file_manager_commands.

BUG=636289
TEST=run_compiler
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2846163002
Cr-Commit-Position: refs/heads/master@{#467917}