cc/paint: Make PaintImage::GetSkImageForFrame private.

This was public because GpuImageDecodeCache used skia to get the decode,
which is not required anymore. Going forward we should eliminate this
within PaintImage itself.

R=vmpstr@chromium.org

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: Id34affc108d4f13f6aae53c19da61a884823b5b9
Reviewed-on: https://chromium-review.googlesource.com/779768Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519958}

cc/paint/paint_image.h

@@ -166,9 +166,6 @@ class CC_PAINT_EXPORT PaintImage {
   // Returns the total number of frames known to exist in this image.
   size_t FrameCount() const;
 
-  // Returns an SkImage for the frame at |index|.
-  sk_sp<SkImage> GetSkImageForFrame(size_t index) const;
-
   std::string ToString() const;
 
  private:
@@ -187,6 +184,9 @@ class CC_PAINT_EXPORT PaintImage {
   void CreateSkImage();
   PaintImage MakeSubset(const gfx::Rect& subset) const;
 
+  // Returns an SkImage for the frame at |index|.
+  sk_sp<SkImage> GetSkImageForFrame(size_t index) const;
+
   sk_sp<SkImage> sk_image_;
 
   sk_sp<PaintRecord> paint_record_;