Reland crrev.com/c/962863 with a build fix for an issue identified by the clobber build.

components/proximity_auth: Make this component CrOS-only.

This is true in practice.

R=khorimoto@chromium.org, sky@chromium.org

Bug: none
Test: none
Change-Id: Ibc90d3d78fc6e7829a8fc282d1fd8bd9763d6fc1
Reviewed-on: https://chromium-review.googlesource.com/963696
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543285}

• BUILD.gn[diff]
• chrome/browser/extensions/BUILD.gn[diff]
• chrome/browser/ui/BUILD.gn[diff]
• chrome/browser/ui/webui/chrome_web_ui_controller_factory.cc[diff]
• chrome/browser/ui/webui/signin/user_manager_screen_handler.cc[diff]
• components/BUILD.gn[diff]
• components/proximity_auth/BUILD.gn[diff]
• components/proximity_auth/screenlock_bridge.cc[diff]
• components/proximity_auth/screenlock_bridge.h[diff]
• components/proximity_auth/unlock_manager_impl.cc[diff]
• components/proximity_auth/unlock_manager_impl.h[diff]
• components/proximity_auth/unlock_manager_impl_unittest.cc[diff]
• components/proximity_auth/webui/BUILD.gn[diff]