Sk_API for SkImageGenerator and SkInstallDiscardablePixelRef

Added SK_API to SkImageGenerator (already in include/).

Moved SkDiscardablePixelRef::Install to SkInstallDiscardablePixelRef,
added SK_API to that function, and moved declaration to
SkImageGenerator.h

This keeps the SkDiscardablePixelRef internal to Skia, but exposes a
method to install it into a bitmap.

Modifed tests that rely on this functio to use new version.

BUG=
R=mtklein@google.com, reed@google.com

Review URL: https://codereview.chromium.org/111713002

git-svn-id: http://skia.googlecode.com/svn/trunk@12612 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index 1e4ab3f..f97e893 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -9,7 +9,6 @@
 #include "SkCanvas.h"
 #include "SkData.h"
 #include "SkDecodingImageGenerator.h"
-#include "SkDiscardablePixelRef.h"
 #include "SkDiscardableMemoryPool.h"
 #include "SkImageDecoder.h"
 #include "SkCachingPixelRef.h"
@@ -257,7 +256,7 @@
         // Ignore factory; use global SkScaledImageCache.
         success = SkCachingPixelRef::Install(gen.detach(), &lazy);
     } else {
-        success = SkDiscardablePixelRef::Install(gen.detach(), &lazy, factory);
+        success = SkInstallDiscardablePixelRef(gen.detach(), &lazy, factory);
     }
     REPORTER_ASSERT(reporter, success
                     == (TestImageGenerator::kFailGetInfo_TestType != type));