Add AlphaThreshold filter.

This is based on the Bitmap Alpha Threshold filter, and will be used by Chromium
to implement the window shape API.

R=bsalomon@chromium.org, wez@chromium.org, bsalomon@google.com, reed@google.com

Author: zork@chromium.org

Review URL: https://codereview.chromium.org/115633002

git-svn-id: http://skia.googlecode.com/svn/trunk@12935 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp/public_headers.gypi b/gyp/public_headers.gypi
index a9ef4dd..b42623f 100644
--- a/gyp/public_headers.gypi
+++ b/gyp/public_headers.gypi
@@ -72,6 +72,7 @@
       'effects/SkMorphologyImageFilter.h',
       'effects/Sk2DPathEffect.h',
       'effects/SkXfermodeImageFilter.h',
+      'effects/SkAlphaThresholdFilter.h',
       'effects/SkArithmeticMode.h',
       'effects/SkMergeImageFilter.h',
       'effects/SkPerlinNoiseShader.h',