Add ETC1 format to SkTextureCompressor

src/utils/SkTextureCompressor.h

Index: src/utils/SkTextureCompressor.h
diff --git a/src/utils/SkTextureCompressor.h b/src/utils/SkTextureCompressor.h
index eac8c5eea397617ceb6d6299b0ec2a0412a5f98c..4254ae76fb902272eaabfae59d7940bb3499529e 100644
--- a/src/utils/SkTextureCompressor.h
+++ b/src/utils/SkTextureCompressor.h
@@ -18,9 +18,18 @@ namespace SkTextureCompressor {
     // Various texture compression formats that we support.
     enum Format {
         // Alpha only formats.
-        kLATC_Format,       // 4x4 blocks, compresses A8
-        kR11_EAC_Format,    // 4x4 blocks, compresses A8
-        kASTC_12x12_Format, // 12x12 blocks, compresses A8
+        kLATC_Format,       // 4x4 blocks, (de)compresses A8
+        kR11_EAC_Format,    // 4x4 blocks, (de)compresses A8
+
+        // RGB only formats
+        kETC1_Format,       // 4x4 blocks, compresses RGB 565, decompresses 8-bit RGB
+                            //    NOTE: ETC1 supports 8-bit RGB compression, but we
+                            //    currently don't have any RGB8 SkColorTypes. We could
+                            //    support 8-bit RGBA but we would have to preprocess the
+                            //    bitmap to insert alphas.
+
+        // Multi-purpose formats
+        kASTC_12x12_Format, // 12x12 blocks, compresses A8, decompresses RGBA
 
         kLast_Format = kASTC_12x12_Format
     };
@@ -48,11 +57,7 @@ namespace SkTextureCompressor {
     // destination buffer. The width and height of the data passed corresponds
     // to the width and height of the uncompressed image. The destination buffer (dst)
     // is assumed to be large enough to hold the entire decompressed image. The
-    // decompressed image colors are determined based on the passed format:
-    //
-    // LATC -> Alpha 8
-    // R11_EAC -> Alpha 8
-    // ASTC -> RGBA
+    // decompressed image colors are determined based on the passed format.
     //
     // Note, CompressBufferToFormat compresses A8 data into ASTC. However,
     // general ASTC data encodes RGBA data, so that is what the decompressor