change SkColorTable to be immutable

src/gpu/SkGr.cpp

Index: src/gpu/SkGr.cpp
diff --git a/src/gpu/SkGr.cpp b/src/gpu/SkGr.cpp
index 7505d7527a1ccd69be37ab41f4889234db6af40a..0b60ede8361ac80febc27ebd0c96b45b49d4f044 100644
--- a/src/gpu/SkGr.cpp
+++ b/src/gpu/SkGr.cpp
@@ -31,7 +31,7 @@ static void build_compressed_data(void* buffer, const SkBitmap& bitmap) {
     char* dst = (char*)buffer;
 
     memcpy(dst, ctable->lockColors(), ctable->count() * sizeof(SkPMColor));
-    ctable->unlockColors(false);
+    ctable->unlockColors();
 
     // always skip a full 256 number of entries, even if we memcpy'd fewer
     dst += kGrColorTableSize;