device scale factor detection: use gtk-xft-dpi consistently

Before this CL, we used either gtk-xft-dpi (UI fonts) or the physical DPI of
a more or less random display. This CL consistently uses gtk-xft-dpi which
is the better alternative: users can directly influence it (by changing the
Xft.dpi X resource, or via their gtkrc).

See also https://github.com/derat/font-config-info for displaying your
current system settings.

This is a re-do of https://codereview.chromium.org/1070433002/, but with
https://codereview.chromium.org/1084453002/ applied (fixes content_shell).

BUG=473089, 143619

Review URL: https://codereview.chromium.org/1085603002

Cr-Commit-Position: refs/heads/master@{#324928}
4 files changed