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/bench/nanobench.cpp b/bench/nanobench.cpp
index 6c60aa8..e67415e 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -630,7 +630,6 @@
         if (!FLAGS_simpleCodec) {
             fColorTypes.push_back(kRGB_565_SkColorType);
             fColorTypes.push_back(kAlpha_8_SkColorType);
-            fColorTypes.push_back(kIndex_8_SkColorType);
             fColorTypes.push_back(kGray_8_SkColorType);
         }
     }
@@ -859,13 +858,8 @@
                 const size_t rowBytes = info.minRowBytes();
                 SkAutoMalloc storage(info.getSafeSize(rowBytes));
 
-                // Used if fCurrentColorType is kIndex_8_SkColorType
-                int colorCount = 256;
-                SkPMColor colors[256];
-
                 const SkCodec::Result result = codec->getPixels(
-                        info, storage.get(), rowBytes, nullptr, colors,
-                        &colorCount);
+                        info, storage.get(), rowBytes);
                 switch (result) {
                     case SkCodec::kSuccess:
                     case SkCodec::kIncompleteInput: