Use enclosing rect to find WindowTreeHost size.

This was using the floored size, so with fractional device scale factors
the right and bottom pixels sometimes aren't drawn. The resulting black
area looks ugly with opaque windows (e.g. with a theme on the browser
window).

BUG=607993

Review-Url: https://codereview.chromium.org/2263453002
Cr-Commit-Position: refs/heads/master@{#413515}
3 files changed