Reland "Remove support for decoding to kIndex_8"

Original change's description:
> > Remove support for decoding to kIndex_8
> > 
> > Fix up callsites, and remove tests that no longer make sense.
> > 
> > Bug: skia:6828
> > Change-Id: I2548c4b7528b7b1be7412563156f27b52c9d4295
> > Reviewed-on: https://skia-review.googlesource.com/21664
> > Reviewed-by: Derek Sollenberger <djsollen@google.com>
> > Commit-Queue: Leon Scroggins <scroggo@google.com>
> 
> TBR=djsollen@google.com,scroggo@google.com
> 
> Change-Id: I1bc669441f250690884e75a9a61427fdf75c6907
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:6828
> Reviewed-on: https://skia-review.googlesource.com/22120
> Reviewed-by: Leon Scroggins <scroggo@google.com>
> Commit-Queue: Leon Scroggins <scroggo@google.com>

TBR=djsollen@google.com,scroggo@google.com

Bug: skia:6828
Change-Id: I36ff5a11c529d29e8adc95f43b8edc6fd1dbf5b8
Reviewed-on: https://skia-review.googlesource.com/22320
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
diff --git a/tests/CodecAnimTest.cpp b/tests/CodecAnimTest.cpp
index 27fe003..84dc9fa 100644
--- a/tests/CodecAnimTest.cpp
+++ b/tests/CodecAnimTest.cpp
@@ -60,7 +60,7 @@
     options.fPriorFrame = SkCodec::kNone;
 
     const auto result = codec->getPixels(info, bm.getPixels(), bm.rowBytes(),
-                                         &options, nullptr, nullptr);
+                                         &options);
     REPORTER_ASSERT(r, result == SkCodec::kSuccess);
 }
 
@@ -329,7 +329,7 @@
                 opts.fFrameIndex = index;
                 opts.fPriorFrame = cachedIndex;
                 const auto result = codec->getPixels(decodeInfo, bm->getPixels(), bm->rowBytes(),
-                                                     &opts, nullptr, nullptr);
+                                                     &opts);
                 if (cachedIndex != SkCodec::kNone && restore_previous(frameInfos[cachedIndex])) {
                     if (result == SkCodec::kInvalidParameters) {
                         return true;