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This CL adds main_utils.*, which has the framework to dispatch Zucchini
commands and print help messages. This is used by the Zucchini main
program, and is not part of Zucchini library.
- Add ::Command: A struct containing command name (e.g., "-gen",
  "-apply"), help message, and a base::Callback containing code for
  the main command. This allows Zucchini command dispatch without bulky
  bulky class hierarchy and superfluous instantialization.
- Add ::CommandRegistry: A class to manage ::Command instances and
  dispatch commands, or print help messages.
- Add ::ResourceUsageTracker: A class instantiated in main() to track
  resource usage for valid command invocations, and and print them at
  end. Specifically:
  - Zucchini.PeakPagefileUsage (KiB)
  - Zucchini.PeakWorkingSetSize (KiB)
  - Zucchini.TotalTime (s)
  Also add "-quiet" switch to disable this.
- Add stubs for Zucchini-gen and Zucchini-apply, to be populated later.
- Starting to use LOG(INFO) to display output. This requires updating
  PRESUBMIT.py to add Zucchini as an exception.

Bug: 729154
Change-Id: Id5263435100dc73b2d8917ca2bcd04245ed41934
Reviewed-on: https://chromium-review.googlesource.com/567492
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486762}