MD Settings: Replace paper-item with button under iron-dropdown, part1.
Remaining paper-item references will be removed in part2.
BUG=
602896
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
Committed:
https://crrev.com/73d0796f5409a410d37b5fb02c41c7c848f61077
Cr-Commit-Position: refs/heads/master@{#406462}