Add can-ignore-rect hint to clear call

src/effects/SkBitmapAlphaThresholdShader.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 "SkBitmapAlphaThresholdShader.h"
 
 class BATShader : public SkShader {
@@ skipping 223 matching lines @@
     GrAutoScratchTexture ast(context, maskDesc, GrContext::kApprox_ScratchTexMatch);
     GrTexture*  maskTexture = ast.texture();
     if (NULL == maskTexture) {
         return NULL;
     }
 
     GrPaint grPaint;
     grPaint.setBlendFunc(kOne_GrBlendCoeff, kZero_GrBlendCoeff);
     SkRegion::Iterator iter(fRegion);
     context->setRenderTarget(maskTexture->asRenderTarget());
-    context->clear(NULL, 0x0);
+    context->clear(NULL, 0x0, true);
 
     // offset to ensure border is zero on top/left
     SkMatrix matrix;
     matrix.setTranslate(SK_Scalar1, SK_Scalar1);
     context->setMatrix(matrix);
 
     while (!iter.done()) {
         SkRect rect = SkRect::MakeFromIRect(iter.rect());
         context->drawRect(grPaint, rect);
         iter.next();
@@ skipping 15 matching lines @@
     GrEffectRef* effect = ThresholdEffect::Create(bmpTexture, bmpMatrix,
                                                   maskTexture, maskMatrix,
                                                   fThreshold);
 
     GrUnlockAndUnrefCachedBitmapTexture(bmpTexture);
 
     return effect;
 }
 
 #endif