Fix tab drag and drop with hi-dpi.

This change converts between DIP and pixels in a few places that were
not converted before, but are necessary for drag & drop of tabs to work
correctly.

R=sadrul@chromium.org
BUG=458804,143619

Review URL: https://codereview.chromium.org/1003863005

Cr-Commit-Position: refs/heads/master@{#321787}
4 files changed