Revert AlphaThresholdFilter to its old behavior wrt premul

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 
 #include "SkBitmap.h"
@@ skipping 394 matching lines @@
         return nullptr;
     }
 
 
     SkMatrix localInverse;
     if (!ctx.ctm().invert(&localInverse)) {
         return nullptr;
     }
 
     SkImageInfo info = SkImageInfo::MakeN32(bounds.width(), bounds.height(),
-                                            inputBM.alphaType());
+                                            kPremul_SkAlphaType);
 
     SkBitmap dst;
     if (!dst.tryAllocPixels(info)) {
         return nullptr;
     }
 
     SkAutoLockPixels dstLock(dst);
 
     U8CPU innerThreshold = (U8CPU)(fInnerThreshold * 0xFF);
     U8CPU outerThreshold = (U8CPU)(fOuterThreshold * 0xFF);
@@ skipping 39 matching lines @@
                                           dst);
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
     str->appendf("SkAlphaThresholdImageFilter: (");
     str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
     str->append(")");
 }
 #endif