Reland "Add sRGB 8888 colortype"

This is a reland of 0f7c10ef5681d5c739bcd8862c58d856fd663a0c

Original change's description:
> Add sRGB 8888 colortype
>
> A color type that linearizes just after loading, and re-encodes to sRGB
> just before storing, mimicking the GPU formats that work the same way.
>
> Notes:
>   - No mipmap support
>   - No SkPngEncoder support (HashAndEncode's .pngs are ok, though?)
>   - Needs better testing
>
> This is a re-creation of reviews.skia.org/392990
>
> Change-Id: I4739c2280211e7176aae98ba0a8476a7fe5efa72
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/438219
> Commit-Queue: Brian Osman <brianosman@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

Change-Id: I5b6bb28c4c1faa6c97fcad7552d12c331535714d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/441402
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/tools/ToolUtils.cpp b/tools/ToolUtils.cpp
index 1eed400..681ac22 100644
--- a/tools/ToolUtils.cpp
+++ b/tools/ToolUtils.cpp
@@ -55,6 +55,7 @@
         case kRGB_565_SkColorType:            return "RGB_565";
         case kARGB_4444_SkColorType:          return "ARGB_4444";
         case kRGBA_8888_SkColorType:          return "RGBA_8888";
+        case kSRGBA_8888_SkColorType:         return "SRGBA_8888";
         case kRGB_888x_SkColorType:           return "RGB_888x";
         case kBGRA_8888_SkColorType:          return "BGRA_8888";
         case kRGBA_1010102_SkColorType:       return "RGBA_1010102";
@@ -83,6 +84,7 @@
         case kRGB_565_SkColorType:            return "565";
         case kARGB_4444_SkColorType:          return "4444";
         case kRGBA_8888_SkColorType:          return "8888";
+        case kSRGBA_8888_SkColorType:         return "8888";
         case kRGB_888x_SkColorType:           return "888";
         case kBGRA_8888_SkColorType:          return "8888";
         case kRGBA_1010102_SkColorType:       return "1010102";