Bug 1439857 - Fix too big fonts when devPixelsPerPx is set; r=stransky

This fixes regression introduced by hidpi support for wayland in
case devPixelsPerPx is set (bug 1431337).

MozReview-Commit-ID: DOh0aTcxbVG

--HG--
extra : rebase_source : ac3ac449a99e0123b8a64ea36f25d312bb3c4483

browser/base/content/test/performance/hidpi/browser.ini

@@ -3,4 +3,4 @@ prefs =
   layout.css.devPixelsPerPx='2'
 
 [../browser_startup_images.js]
-skip-if = !debug || ((os == 'win' && (os_version == '6.1')) || os == 'linux')  # hidpi results in the toolbar overflowing on Win 7 and Linux
+skip-if = !debug || (os == 'win' && (os_version == '6.1'))  # hidpi results in the toolbar overflowing on Win 7

widget/gtk/nsLookAndFeel.cpp

@@ -790,7 +790,7 @@ nsLookAndFeel::GetFontImpl(FontID aID, nsString& aFontName,
   // Scale the font for the current monitor
   double scaleFactor = nsIWidget::DefaultScaleOverride();
   if (scaleFactor > 0) {
-    aFontStyle.size *= aDevPixPerCSSPixel;
+    aFontStyle.size *= mozilla::widget::ScreenHelperGTK::GetGTKMonitorScaleFactor();
   } else {
     // Remove effect of font scale because it has been already applied in
     // GetSystemFontInfo

widget/gtk/nsNativeThemeGTK.cpp

@@ -80,8 +80,8 @@ GetMonitorScaleFactor(nsIFrame* aFrame)
         return rootWidget->GetDefaultScale().scale / gfxPlatformGtk::GetFontScaleFactor();
     }
   }
-  // We cannot return zero scale because that would lead to divide by zero
-  return (scale < 1) ? 1 : int(round(scale));
+  // Use monitor scaling factor where devPixelsPerPx is set
+  return ScreenHelperGTK::GetGTKMonitorScaleFactor();
 }
 
 nsNativeThemeGTK::nsNativeThemeGTK()