DevTools: [Media Inspector] make both labels on MQ-markers sticky

This patch makes both labels "sticky", so that they always remain on-screen
when MediaInspectorView width gets resized.

See screenshot in the bug.

BUG=382996
R=dgozman@chromium.org

Review URL: https://codereview.chromium.org/346843005

git-svn-id: svn://svn.chromium.org/blink/trunk@176607 bbb929c8-8fbe-4397-9dbb-9b2b20218538