DevTools: [Styles] Make search/filter input placeholders visible

R=pfeldman@chromium.org, pfeldman
BUG=361014

Review URL: https://codereview.chromium.org/237743004

git-svn-id: svn://svn.chromium.org/blink/trunk@171755 bbb929c8-8fbe-4397-9dbb-9b2b20218538