Make surface GetFormat pure virtual, add missing overrides.

ui/gl/gl_surface_format.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GL_GL_SURFACE_FORMAT_H_
 #define UI_GL_GL_SURFACE_FORMAT_H_
 
 #include "ui/gl/gl_export.h"
 
 namespace gl {
 
+// Expresses surface format properties that may vary depending
+// on the underlying gl_surface implementation or specific usage
+// scenarios. Intended usage is to support copying formats and
+// checking compatibility.
 class GL_EXPORT GLSurfaceFormat {
  public:
 
   // For use with constructor.
   enum SurfacePixelLayout {
     PIXEL_LAYOUT_DONT_CARE = -1,
     PIXEL_LAYOUT_BGRA,
     PIXEL_LAYOUT_RGBA,
   };
 
+  // Default surface format for the underlying gl_surface subtype.
+  // Use the setters below to change attributes if needed.
   GLSurfaceFormat();
 
+  // Use a specified pixel layout, cf. gl_surface_osmesa.
+  GLSurfaceFormat(SurfacePixelLayout layout);
+
+  // Copy constructor from pre-existing format.
+  GLSurfaceFormat(const GLSurfaceFormat& other);
+
   ~GLSurfaceFormat();
 
-  GLSurfaceFormat(SurfacePixelLayout layout);
-
-  GLSurfaceFormat(const GLSurfaceFormat& other);
-
+  // Helper method to determine if the format is unchanged from the
+  // default at creation time. TODO(klausw): can this be removed?
   bool IsDefault();
 
+  // Surfaceless appears as a format property for backwards
+  // compatibility with the previous enum-based implementation.
+  // TODO(klausw): consider removing it and/or merging it into the
+  // pre-existing IsSurfaceless() predicate for the various Surface
+  // subclasses?
   void SetIsSurfaceless();
   bool IsSurfaceless();
 
+  // A given pair of surfaces is considered compatible if glSetSurface
+  // can be used to switch between them without generating BAD_MATCH
+  // errors, visual errors, or gross inefficiency, and incompatible
+  // otherwise. For example, a pixel layout mismatch would be
+  // considered incompatible. This comparison only makes sense within
+  // the context of a single gl_surface subtype.
   bool IsCompatible(GLSurfaceFormat other_format);
 
   // Default pixel format is RGBA8888. Use this method to select
   // a preference of RGB565. TODO(klausw): use individual setter
   // methods if there's a use case for them.
   void SetRGB565();
 
   // Other properties for future use.
   void SetDepthBits(int depth_bits);
   int GetDepthBits();
 
   void SetStencilBits(int stencil_bits);
   int GetStencilBits();
 
   void SetSamples(int samples);
   int GetSamples();
 
   void SetDefaultPixelLayout(SurfacePixelLayout layout);
 
   SurfacePixelLayout GetPixelLayout();
 
+  // Compute number of bits needed for storing one pixel, not
+  // including any padding. At this point mainly used to distinguish
+  // RGB565 (16) from RGBA8888 (32).
   int GetBufferSize();
 
  private:
   bool is_default_ = true;
   SurfacePixelLayout pixel_layout_ = PIXEL_LAYOUT_DONT_CARE;
   bool is_surfaceless_ = false;
   int red_bits_ = -1;
   int green_bits_ = -1;
   int blue_bits_ = -1;
   int alpha_bits_ = -1;
   int depth_bits_ = -1;
   int samples_ = -1;
   int stencil_bits_ = -1;
 };
 
 }  // namespace gl
 
 #endif  // UI_GL_GL_SURFACE_FORMAT_H_