Make surface GetFormat pure virtual, add missing overrides.

ui/gl/gl_surface_format.h

Index: ui/gl/gl_surface_format.h
diff --git a/ui/gl/gl_surface_format.h b/ui/gl/gl_surface_format.h
index cb7d51d7baaa0db43b655cdb1f0c909031a90257..2fcf504f887b516cf9cb92054a8026910ac9a69d 100644
--- a/ui/gl/gl_surface_format.h
+++ b/ui/gl/gl_surface_format.h
@@ -9,6 +9,10 @@
 
 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:
 
@@ -19,19 +23,36 @@ class GL_EXPORT GLSurfaceFormat {
     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
@@ -53,6 +74,9 @@ class GL_EXPORT GLSurfaceFormat {
 
   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: