Fix up and rename VisibleMargins() in preparation for making use of it.

ui/gfx/image/image_util.h

 // Copyright (c) 2012 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_GFX_IMAGE_IMAGE_UTIL_H_
 #define UI_GFX_IMAGE_IMAGE_UTIL_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 15 matching lines @@
 // Fills the |dst| vector with JPEG-encoded bytes of the 1x representation of
 // the given image.
 // Returns true if the image has a 1x representation and the 1x representation
 // was encoded successfully.
 // |quality| determines the compression level, 0 == lowest, 100 == highest.
 // Returns true if the Image was encoded successfully.
 GFX_EXPORT bool JPEG1xEncodedDataFromImage(const Image& image,
                                            int quality,
                                            std::vector<unsigned char>* dst);
 
-// Returns the visible (non-transparent) width of the 1x rep of the given
-// image. If the image has no transparency, the leading value will be 0 and
-// the trailing will be the image width. Return values are in the 1x width
-// pixel units. Margins are given in 0-based column format. So if the image
-// has only transparent pixels in columns 0, 1, 2, 3, then the leading value
-// will be 4. Similarly, if there are all transparent pixels in column
-// width-2, width-1, then the trailing margin value will be width-3.
-// Returns true if the value is computed from opacity, false if it is a
-// default value because of null image, missing Rep, etc.
-// This method is only suitable for fairly small images (i.e. 16x16).
-// The margins for a completely transparent image will be w/2-1, w/2, but this
-// will be an expensive operation: it isn't expected that it will be frequently
-// calculated.
-GFX_EXPORT bool VisibleMargins(const ImageSkia& image,
-                               int* leading, int* trailing);
+// Computes the width of any nearly-transparent regions at the sides of the
+// image and returns them in |left| and |right|.  This checks each column of
+// pixels from the outsides in, looking for anything with alpha above a
+// reasonably small value.  For a fully-opaque image, the margins will thus be
+// (0, 0); for a fully-transparent image, the margins will be
+// (width / 2, width / 2), with |left| getting the extra pixel for odd widths.
+GFX_EXPORT void GetVisibleMargins(const ImageSkia& image,
+                                  int* left,
+                                  int* right);
 
 }  // namespace gfx
 
 #endif  // UI_GFX_IMAGE_IMAGE_UTIL_H_