Draft at https://codereview.chromium.org/1921133002/.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b6a59711024115cb14728ca0dfdc2a1d507fa567

commit b6a59711024115cb14728ca0dfdc2a1d507fa567
Author: varkha <varkha@chromium.org>
Date: Thu Apr 28 00:50:41 2016

Adds ash::MaterialDesignController and a run-time flag

Adds "ash-enable-md" and "ash-disable-md" flags to enable / disable
Material Design in Chrome OS system UI.
ash::MaterialDesignController::IsMaterial() can be used to query the
mode at run-time.

BUG= 605644 

Review-Url: https://codereview.chromium.org/1921133002
Cr-Commit-Position: refs/heads/master@{#390260}

[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/ash.gyp
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/ash_switches.cc
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/ash_switches.h
[add] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/material_design/material_design_controller.cc
[add] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/material_design/material_design_controller.h
[add] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/material_design/material_design_controller_unittest.cc
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/test/ash_test_helper.cc
[add] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/test/material_design_controller_test_api.cc
[add] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/ash/test/material_design_controller_test_api.h
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/chrome/app/generated_resources.grd
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/chrome/browser/about_flags.cc
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/chrome/browser/chrome_browser_main.cc
[modify] https://crrev.com/b6a59711024115cb14728ca0dfdc2a1d507fa567/tools/metrics/histograms/histograms.xml

Can now use ash::MaterialDesignController::IsMaterial() or ash::MaterialDesignController::IsMaterialExperimental() to guard the MD features / assets / layout.

Description correction for #3, the flag is actually "--ash-md". HT to yiyix@.
chrome://flags/#ash-md

Verified on 8270.0.0 canary.

Related:  Issue 608353