Fixing warning on using dpi in media query.

Since dpi values commonly used as a fallback if value is overriden by dppx-value.

BUG=336276

Review URL: https://codereview.chromium.org/190913002

git-svn-id: svn://svn.chromium.org/blink/trunk@168846 bbb929c8-8fbe-4397-9dbb-9b2b20218538