Remove unused param to SkGIFFrameContext ctor

After updating the transparency index code, the reader parameter to
SkGIFFrameContext's ctor is no longer needed. This patch removes it.

BUG=skia:7069

Change-Id: If129f825639e8d43d73794adca2de09785f56a3c
Reviewed-on: https://skia-review.googlesource.com/52602
Commit-Queue: Chris Blume <cblume@chromium.org>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
diff --git a/third_party/gif/SkGifImageReader.h b/third_party/gif/SkGifImageReader.h
index d1063dd..b5eca10 100644
--- a/third_party/gif/SkGifImageReader.h
+++ b/third_party/gif/SkGifImageReader.h
@@ -196,7 +196,7 @@
 // LocalFrame output state machine.
 class SkGIFFrameContext : public SkFrame {
 public:
-    SkGIFFrameContext(SkGifImageReader* reader, int id)
+    SkGIFFrameContext(int id)
         : INHERITED(id)
         , m_transparentPixel(SkGIFColorMap::kNotFound)
         , m_dataSize(0)