Make BRD take advantage of zero initialized memory

tools/codec/SkBitmapRegionCanvas.cpp

Index: tools/codec/SkBitmapRegionCanvas.cpp
diff --git a/tools/codec/SkBitmapRegionCanvas.cpp b/tools/codec/SkBitmapRegionCanvas.cpp
index ffcab9596a95f109d577b3f47e393bac6ab7d723..f0dcf6a19cdbb833aa11478c966089a5be9beb50 100644
--- a/tools/codec/SkBitmapRegionCanvas.cpp
+++ b/tools/codec/SkBitmapRegionCanvas.cpp
@@ -15,7 +15,7 @@ SkBitmapRegionCanvas::SkBitmapRegionCanvas(SkCodec* decoder)
     , fDecoder(decoder)
 {}
 
-bool SkBitmapRegionCanvas::decodeRegion(SkBitmap* bitmap, SkBitmap::Allocator* allocator,
+bool SkBitmapRegionCanvas::decodeRegion(SkBitmap* bitmap, SkBRDAllocator* allocator,
         const SkIRect& desiredSubset, int sampleSize, SkColorType dstColorType,
         bool requireUnpremul) {
     // Reject color types not supported by this method
@@ -102,10 +102,10 @@ bool SkBitmapRegionCanvas::decodeRegion(SkBitmap* bitmap, SkBitmap::Allocator* a
     // TODO (msarett): Can we make this faster by implementing it to only
     //                 zero parts of the image that we won't overwrite with
     //                 pixels?
-    // TODO (msarett): This could be skipped if memory is zero initialized.
-    //                 This would matter if this code is moved to Android and
-    //                 uses Android bitmaps.
-    if (SubsetType::kPartiallyInside_SubsetType == type) {
+    SkCodec::ZeroInitialized zeroInit = allocator ? allocator->zeroInit() :

[scroggo, 2015/11/10 22:31:14]

It looks like we only care about the variable zeroInit inside the if statement.
how about the following:

if (SubsetType::kPartiallyInside_SubsetType == type) {
  zeroInit = ...
  if (no_zeroInit == zeroInit) {
    bitmap->eraseColor(0);
  }
}

[msarett, 2015/11/10 22:49:59]

Done.

+                SkCodec::kNo_ZeroInitialized;
+    if (SubsetType::kPartiallyInside_SubsetType == type &&
+            SkCodec::kNo_ZeroInitialized == zeroInit) {
         bitmap->eraseColor(0);
     }