DevTools: Make hide shortcut work with shadow DOM

Drive-by: replace body element name with html for /deep/ inspector styles.

BUG=406389

Review URL: https://codereview.chromium.org/682653002

git-svn-id: svn://svn.chromium.org/blink/trunk@184474 bbb929c8-8fbe-4397-9dbb-9b2b20218538