Gtk3: More fixes and refactorings

docs/linux_gtk_theme_integration.md

 # Linux GTK Theme Integration
 
 The GTK+ port of Chromium has a mode where we try to match the user's GTK theme
 (which can be enabled under Settings -> Appearance -> Use GTK+ theme).
 
 # GTK3
 
 At some point after version 57, Chromium will switch to using the GTK3 theme by
 default.
 
 ## How Chromium determines which colors to use
 
 GTK3 added a new CSS theming engine which gives fine-tuned control over how
 widgets are styled. Chromium's themes, by contrast, are much simpler: it is
 mostly a list of about 80 colors (see //src/ui/native_theme/native_theme.h)
 overridden by the theme. Chromium usually doesn't use GTK to render entire
 widgets, but instead tries to determine colors from them.
 
-There are three types of colors Chromium needs from widgets: 
-
-* Foreground color: determined by the CSS "color" property
-* Background color: determined by the CSS "background-color" and "background-image" properties
-* Border color: determined by the "border-color", "border-image",
-  "border-style", and "border-width" properties
-
-Backgrounds and borders are complicated because in general they might have
-multiple gradients or images. To get the color, Chromium uses GTK to render the
-background or border into a single pixel and uses the resulting color for
-theming. This mostly gives reasonable results, but in case theme authors do not
-like the resulting color, they have the option to theme Chromium widgets
-specially.
+Chromium needs foreground, background and border colors from widgets.  The
+foreground color is simply taken from the CSS "color" property.  Backgrounds and
+borders are complicated because in general they might have multiple gradients or
+images. To get the color, Chromium uses GTK to render the background or border
+into a 24x24 bitmap and uses the average color for theming. This mostly gives
+reasonable results, but in case theme authors do not like the resulting color,
+they have the option to theme Chromium widgets specially.
 
 ## Note to GTK theme authors: How to theme Chromium widgets
 
 Every widget Chromium uses will have a "chromium" style class added to it. For
-example, a texfield selector might look like:
+example, a textfield selector might look like:
 
 ```
 .window.background.chromium .entry.chromium
 ```
 
 If themes want to handle Chromium textfields specially, for GTK3.0 - GTK3.19,
 they might use:
 
 ```
 /* Normal case */
 .entry {
     color: #ffffff;
     background-color: #000000;
 }
 
 /* Chromium-specific case */
 .entry.chromium {
     color: #ff0000;
     background-color: #00ff00;
 }
 ```
 
 For GTK3.20 or later, themes will as usual have to replace ".entry" with
 "entry".
 
-Additional requirements for border colors to be picked up:
-
-* Must have a border-style that is not none.
-* Must have a border-width that is nonzero.
-
 The list of CSS selectors that Chromium uses to determine its colors is in
 //src/chrome/browser/ui/libgtkui/native_theme_gtk3.cc.
 
 # GTK2
 
 Chromium's GTK2 theme will soon be deprecated, and this section will be removed.
 
 ## Describing the previous heuristics
 
 The heuristics often don't pick good colors due to a lack of information in the
@@ skipping 112 matching lines @@
 
 `MetaFrames` is a class that was used in metacity to communicate color
 information to the window manager. During the Hardy Heron days, we slurped up
 the data and used it as a key part of our heuristics. At least on my Lucid Lynx
 machine, none of the GNOME GTK+ themes have `MetaFrames` styling. (As mentioned
 above, several of the XFCE themes do, though.)
 
 Internally to chrome, our `ChromeGtkFrame` class inherits from `MetaFrames`
 (again, which inherits from `GtkWindow`) so any old themes that style the
 `MetaFrames` class are backwards compatible.