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

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, 498426
Review URL: https://codereview.chromium.org/1085603002

Cr-Commit-Position: refs/heads/master@{#324928}
(cherry picked from commit d459a5c49a02507b9ac77ffbbf33dffeb3bc9c34)

TBR=laforge@chromium.org, oshima@chromium.org

Committed: https://chromium.googlesource.com/chromium/src/+/da20b3fc0aed6674160b02eba4919f6ec2379ec7

[Michael Moss, 2015-06-10 20:30:58]

mmoss@chromium.org changed reviewers:
+ laforge@chromium.org, oshima@chromium.org
- laforge@google.com, oshima@google.com

[laforge, 2015-06-10 20:39:02]

On 2015/06/10 20:26:39, Michael Moss wrote:

LGTM

[Michael Moss, 2015-06-10 20:40:30]

Committed patchset #1 (id:1) manually as
da20b3fc0aed6674160b02eba4919f6ec2379ec7 (presubmit successful).