Move SkDecodingImageGenerator.h to include/

This will allow Android to access it and remove SkImageRef.

Depends on https://codereview.chromium.org/293283002/

BUG=skia:2389
R=reed@google.com, djsollen@google.com, halcanary@google.com

Author: scroggo@google.com

Review URL: https://codereview.chromium.org/228613003

git-svn-id: http://skia.googlecode.com/svn/trunk@14891 2bbb7eff-a529-9590-31e7-b0007b416f81

gyp/images.gyp

@@ -31,6 +31,7 @@
         '../src/images/',
       ],
       'sources': [
+        '../include/images/SkDecodingImageGenerator.h',
         '../include/images/SkForceLinking.h',
         '../include/images/SkImageRef.h',
         '../include/images/SkImageRef_GlobalPool.h',
@@ -42,7 +43,6 @@
         '../src/images/bmpdecoderhelper.h',
 
         '../src/images/SkDecodingImageGenerator.cpp',
-        '../src/images/SkDecodingImageGenerator.h',
         '../src/images/SkForceLinking.cpp',
         '../src/images/SkImageDecoder.cpp',
         '../src/images/SkImageDecoder_FactoryDefault.cpp',