Chromium change:

https://chromium.googlesource.com/chromium/src/+/bdb2e80550099f087aa5af29eaba1121bc1c1c62

commit bdb2e80550099f087aa5af29eaba1121bc1c1c62
Author: dpapad <dpapad@chromium.org>
Date:   Thu Jun 27 16:32:57 2019 +0000

    WebUI: Move md_select_css.html from html/ to cr_elements/

    This is done for two reasons.
    1) Consistency: md_select_css.html and action_link_css.html are the only
       <dom-module>s that don't reside under cr_elements/ or
       cr_components/. Other shared styles (like hidden_style_css.html) are
       already under cr_elements/.
    2) For the purposes of auto-generating Polymer3 files, moving those files
       over eliminates a corner case (with regards to path rewriting) that
       otherwise needs to be handled by polymer_modulizer().

    Also note that |flattenhtml| was unnecessarily used for md_select_css.html,
    removed as part of the move.

    action_link_css.html will be moved in a separate CL.

    Bug: 965770