Revert "Revert "cache SkImage::Info calculation in lazypixelref""

This reverts commit 81eba32ab10f9210c742938819cf1218be5611c9.

ImageDecoder is changed to allow info to be NULL, since it is an output-only parameter.

R=scroggo@google.com

Review URL: https://codereview.chromium.org/33573002

git-svn-id: http://skia.googlecode.com/svn/trunk@11896 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageDecoder.cpp b/src/images/SkImageDecoder.cpp
index 3ee4112..89bd059 100644
--- a/src/images/SkImageDecoder.cpp
+++ b/src/images/SkImageDecoder.cpp
@@ -405,10 +405,6 @@
 bool SkImageDecoder::DecodeMemoryToTarget(const void* buffer, size_t size,
                                           SkImage::Info* info,
                                           const SkBitmapFactory::Target* target) {
-    if (NULL == info) {
-        return false;
-    }
-
     // FIXME: Just to get this working, implement in terms of existing
     // ImageDecoder calls.
     SkBitmap bm;
@@ -427,14 +423,17 @@
     // Now set info properly.
     // Since Config is SkBitmap::kARGB_8888_Config, SkBitmapToImageInfo
     // will always succeed.
-    SkAssertResult(SkBitmapToImageInfo(bm, info));
+    if (info) {
+        SkAssertResult(SkBitmapToImageInfo(bm, info));
+    }
 
     if (NULL == target) {
         return true;
     }
 
     if (target->fRowBytes != SkToU32(bm.rowBytes())) {
-        if (target->fRowBytes < SkImageMinRowBytes(*info)) {
+        size_t minRB = SkBitmap::ComputeRowBytes(bm.config(), bm.width());
+        if (target->fRowBytes < minRB) {
             SkDEBUGFAIL("Desired row bytes is too small");
             return false;
         }