change SkColorTable to be immutable

src/images/SkImageDecoder_libgif.cpp

Index: src/images/SkImageDecoder_libgif.cpp
diff --git a/src/images/SkImageDecoder_libgif.cpp b/src/images/SkImageDecoder_libgif.cpp
index 08f37efced9fc8bdee36b286dc52ad44f06ac028..ab0fbdaf3f777855d2b8621eed7a1c1c4abe7325 100644
--- a/src/images/SkImageDecoder_libgif.cpp
+++ b/src/images/SkImageDecoder_libgif.cpp
@@ -248,10 +248,13 @@ bool SkGIFImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* bm, Mode mode) {
                 if (NULL == cmap) {
                     return error_return(gif, *bm, "null cmap");
                 }
-
                 colorCount = cmap->ColorCount;
-                SkAutoTMalloc<SkPMColor> colorStorage(colorCount);
-                SkPMColor* colorPtr = colorStorage.get();
+                if (colorCount > 256) {
+                    colorCount = 256;   // our kIndex8 can't support more
+                }
+
+                SkPMColor colorPtr[256]; // storage for worst-case
+                SkAlphaType alphaType = kOpaque_SkAlphaType;
                 for (int index = 0; index < colorCount; index++) {
                     colorPtr[index] = SkPackARGB32(0xFF,
                                                    cmap->Colors[index].Red,
@@ -263,10 +266,12 @@ bool SkGIFImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* bm, Mode mode) {
                 bool reallyHasAlpha = transpIndex >= 0;
                 if (reallyHasAlpha) {
                     colorPtr[transpIndex] = SK_ColorTRANSPARENT; // ram in a transparent SkPMColor
+                    alphaType = kPremul_SkAlphaType;
                 }
 
-                SkAutoTUnref<SkColorTable> ctable(SkNEW_ARGS(SkColorTable, (colorPtr, colorCount)));
-                ctable->setIsOpaque(!reallyHasAlpha);
+                SkAutoTUnref<SkColorTable> ctable(SkNEW_ARGS(SkColorTable,
+                                                  (colorPtr, colorCount,
+                                                   alphaType)));
                 if (!this->allocPixelRef(bm, ctable)) {
                     return error_return(gif, *bm, "allocPixelRef");
                 }