change SkColorTable to be immutable

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index 4e4106aede85f8266c3b9ffd234e025f9f6a0e02..48c2b1fb5baddf31367773d352389bd6f65a097f 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -648,8 +648,6 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
     png_colorp palette;
     png_bytep trans;
     int numTrans;
-    bool reallyHasAlpha = false;
-    SkColorTable* colorTable = NULL;
 
     png_get_PLTE(png_ptr, info_ptr, &palette, &numPalette);
 
@@ -660,17 +658,17 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
         the colortable by 1 (if its < 256) and duplicate the last color into that slot.
     */
     int colorCount = numPalette + (numPalette < 256);
+    SkPMColor colorStorage[256];    // worst-case storage
+    SkPMColor* colorPtr = colorStorage;
 
-    colorTable = SkNEW_ARGS(SkColorTable, (colorCount));
-
-    SkPMColor* colorPtr = colorTable->lockColors();
     if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
         png_get_tRNS(png_ptr, info_ptr, &trans, &numTrans, NULL);
         *hasAlphap = (numTrans > 0);
     } else {
         numTrans = 0;
-        colorTable->setFlags(colorTable->getFlags() | SkColorTable::kColorsAreOpaque_Flag);
     }
+    bool reallyHasAlpha = numTrans > 0;
+
     // check for bad images that might make us crash
     if (numTrans > numPalette) {
         numTrans = numPalette;
@@ -703,8 +701,18 @@ bool SkPNGImageDecoder::decodePalette(png_structp png_ptr, png_infop info_ptr,
     if (numPalette < 256) {
         *colorPtr = colorPtr[-1];
     }
-    colorTable->unlockColors(true);
-    *colorTablep = colorTable;
+
+    SkAlphaType alphaType = kOpaque_SkAlphaType;
+    if (reallyHasAlpha) {
+        if (this->getRequireUnpremultipliedColors()) {
+            alphaType = kUnpremul_SkAlphaType;
+        } else {
+            alphaType = kPremul_SkAlphaType;
+        }
+    }
+
+    *colorTablep = SkNEW_ARGS(SkColorTable,
+                              (colorStorage, colorCount, alphaType));
     *reallyHasAlphap = reallyHasAlpha;
     return true;
 }