DevTools: Add a checkbox to enable async stacks in the sidebar header.

This also removes the checkbox from the Settings page.

BUG=272416
R=pfeldman, yurys

Review URL: https://codereview.chromium.org/102833003

git-svn-id: svn://svn.chromium.org/blink/trunk@163448 bbb929c8-8fbe-4397-9dbb-9b2b20218538