Gtk3: Fix fuzzy breeze frame buttons when using scale factors > 1

When using scale factors > 1, it is necessary to set the scale factor on all
GtkStyleContext's so that GTK will fetch higher resolution assets.

This was only seen on the Breeze theme, because it's the only one that uses
bitmap backgrounds.

BUG=794358
R=erg@chromium.org

Change-Id: Ib2c42e4869156caba94f377b583cbac96f44ad98
Reviewed-on: https://chromium-review.googlesource.com/847683Reviewed-by: Elliot Glaysher <erg@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526638}

chrome/browser/ui/libgtkui/gtk_util.cc

@@ -25,6 +25,7 @@
 #include "ui/events/keycodes/keyboard_code_conversion_x.h"
 #include "ui/gfx/color_utils.h"
 #include "ui/gfx/geometry/size.h"
+#include "ui/views/linux_ui/linux_ui.h"
 
 namespace {
 
@@ -53,6 +54,11 @@ void CommonInitFromCommandLine(const base::CommandLine& command_line,
   }
 }
 
+float GetDeviceScaleFactor() {
+  views::LinuxUI* linux_ui = views::LinuxUI::instance();
+  return linux_ui ? linux_ui->GetDeviceScaleFactor() : 1;
+}
+
 }  // namespace
 
 namespace libgtkui {
@@ -446,6 +452,7 @@ ScopedStyleContext AppendCssNodeToStyleContext(GtkStyleContext* context,
     }
     gtk_style_context_set_state(child_context, child_state);
   }
+  gtk_style_context_set_scale(child_context, std::ceil(GetDeviceScaleFactor()));
   gtk_style_context_set_parent(child_context, context);
   gtk_widget_path_unref(path);
   return child_context;