wayland ui scaling: keep font scale as a separate field in DisplayConfig

This is part of the set of changes required to support Desktop
Environment's "Large Text" accessibility setting in Chromium's Wayland
backend. It actually can also be triggered by other system tools, such
as, Gnome Tweaks app as well as gsettings command line tool in Gnome
ecosystem.

Design doc:
https://docs.google.com/document/d/16kwPm_S0dmxe4kSgKY0_PA7aFvkFcftx6AMzNqT8Nfg

This CL starts by adapting LinuxUi's DisplayConfig data struct to store
and expose the font scale value separately, because, unlike ozone/x11
the wayland backend needs to keep it separate from the surface's scale
values. Refer to the DD for further details and rationale.

For now, only GtkUi with GTK4 is supported. Additional support for GTK3
seems technically possible though will be done in a separate CL, if
possible.

R=sky, thomasanderson

Bug: 40856031, 41486578
Change-Id: I9704c269ccffac103ab9f79d02e87368b9379160
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5834921
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1353682}

ui/display/types/display_config.h

diff --git a/ui/display/types/display_config.h b/ui/display/types/display_config.h
index 14457ee..d2b66e4 100644
--- a/ui/display/types/display_config.h
+++ b/ui/display/types/display_config.h
@@ -21,10 +21,12 @@
 
   std::vector<DisplayGeometry> display_geometries;
   float primary_scale = 1.0f;
+  float font_scale = 1.0f;
 
   bool operator==(const DisplayConfig& other) const {
     return display_geometries == other.display_geometries &&
-           primary_scale == other.primary_scale;
+           primary_scale == other.primary_scale &&
+           font_scale == other.font_scale;
   }
 };
 

ui/gtk/gtk_ui.cc

diff --git a/ui/gtk/gtk_ui.cc b/ui/gtk/gtk_ui.cc
index 1d41088..d215ab07 100644
--- a/ui/gtk/gtk_ui.cc
+++ b/ui/gtk/gtk_ui.cc
@@ -1021,6 +1021,7 @@
   }
   config.primary_scale =
       std::max(1, gdk_monitor_get_scale_factor(primary)) * font_scale;
+  config.font_scale = font_scale;
   config.display_geometries.reserve(monitors.size());
   for (GdkMonitor* monitor : monitors) {
     GdkRectangle geometry;