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/src/codec/SkBmpStandardCodec.h b/src/codec/SkBmpStandardCodec.h
index ec3d707..f9ce5c6 100644
--- a/src/codec/SkBmpStandardCodec.h
+++ b/src/codec/SkBmpStandardCodec.h
@@ -47,16 +47,15 @@
 protected:
 
     Result onGetPixels(const SkImageInfo& dstInfo, void* dst,
-                       size_t dstRowBytes, const Options&, SkPMColor*,
-                       int*, int*) override;
+                       size_t dstRowBytes, const Options&,
+                       int*) override;
 
     bool onInIco() const override {
         return fInIco;
     }
 
     SkCodec::Result onPrepareToDecode(const SkImageInfo& dstInfo,
-            const SkCodec::Options& options, SkPMColor inputColorPtr[],
-            int* inputColorCount) override;
+            const SkCodec::Options& options) override;
 
 
     uint64_t onGetFillValue(const SkImageInfo&) const override;
@@ -70,9 +69,8 @@
 
     /*
      * Creates the color table
-     * Sets colorCount to the new color count if it is non-nullptr
      */
-    bool createColorTable(SkColorType colorType, SkAlphaType alphaType, int* colorCount);
+    bool createColorTable(SkColorType colorType, SkAlphaType alphaType);
 
     void initializeSwizzler(const SkImageInfo& dstInfo, const Options& opts);