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Change the default value of the --top-chrome-md
flag to map to 'material' on Chrome OS, and
remain as 'non-material' on all other platforms.

On Chrome OS this will impact the assets, layout,
theming, and visual feedback of mouse/touch
interactions across the browser's UI surfaces,
namely:

* Tabstrip
* Toolbar
* Bookmarks bar
* Omnibox dropdown
* Incognito mode
* Bubbles
* Infobars

This CL does NOT make any changes to UI on other
platforms or non-browser UI on Chrome OS (shelf,
tray, etc). Furthermore, this CL should not
introduce any regressions in functionality.

Please use the label
Proj-MaterialDesign-NativeUI when filing bugs
in crbug.

BUG=581143,575270
TEST=MaterialDesignControllerTest.*

Review URL: https://codereview.chromium.org/1633733004

Cr-Commit-Position: refs/heads/master@{#371609}