Changes behavior of iOS-specific gfx::Image constructor.

Chrome iOS is moving to Objective-C Automatic Reference Counting. Under
it, arguments are supposed to be autoreleased. This CL changes the constructor to retain its argument..
This is a part of a multiple CL setup that will change the Image(UIImage*) constructor from consuming +1 refcount to retaining the argument.

TEST=none
BUG=645125

Review-Url: https://codereview.chromium.org/2500373004
Cr-Commit-Position: refs/heads/master@{#438223}

ui/gfx/image/image.cc

@@ -420,7 +420,7 @@ Image::Image(const ImageSkia& image) {
 }
 
 #if defined(OS_IOS)
-Image::Image(UIImage* image) : Image(image, base::scoped_policy::ASSUME) {}
+Image::Image(UIImage* image) : Image(image, base::scoped_policy::RETAIN) {}
 
 Image::Image(UIImage* image, base::scoped_policy::OwnershipPolicy policy)
     : storage_(new internal::ImageStorage(Image::kImageRepCocoaTouch)) {

ui/gfx/image/image.h

@@ -71,7 +71,7 @@ class GFX_EXPORT Image {
   explicit Image(const ImageSkia& image);
 
 #if defined(OS_IOS)
-  // Does not retain |image|; expects to take ownership.
+  // Retains |image|.
   explicit Image(UIImage* image);
 
   // Retains argument according to |policy|.