Update SkImageFilter's cache to handle SkSpecialImages and add unit test

src/core/SkImageFilterCacheKey.h

Index: src/core/SkImageFilterCacheKey.h
diff --git a/src/core/SkImageFilterCacheKey.h b/src/core/SkImageFilterCacheKey.h
new file mode 100644
index 0000000000000000000000000000000000000000..c96f911246e7ea087f088f25f33f9ae0b0959f63
--- /dev/null
+++ b/src/core/SkImageFilterCacheKey.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkImageFilterCacheKey_DEFINED
+#define SkImageFilterCacheKey_DEFINED
+
+struct SkImageFilter::Cache::Key {
+    Key(const uint32_t uniqueID, const SkMatrix& matrix,
+        const SkIRect& clipBounds, uint32_t srcGenID, const SkIRect& srcSubset)
+        : fUniqueID(uniqueID)
+        , fMatrix(matrix)
+        , fClipBounds(clipBounds)
+        , fSrcGenID(srcGenID)
+        , fSrcSubset(srcSubset) {
+        // Assert that Key is tightly-packed, since it is hashed.
+        static_assert(sizeof(Key) == sizeof(uint32_t) + sizeof(SkMatrix) + sizeof(SkIRect) +
+                                     sizeof(uint32_t) + 4 * sizeof(int32_t),
+                                     "image_filter_key_tight_packing");
+        fMatrix.getType();  // force initialization of type, so hashes match
+    }
+
+    uint32_t fUniqueID;
+    SkMatrix fMatrix;
+    SkIRect fClipBounds;
+    uint32_t fSrcGenID;
+    SkIRect fSrcSubset;
+
+    bool operator==(const Key& other) const {
+        return fUniqueID == other.fUniqueID &&
+               fMatrix == other.fMatrix &&
+               fClipBounds == other.fClipBounds &&
+               fSrcGenID == other.fSrcGenID &&
+               fSrcSubset == other.fSrcSubset;
+    }
+};
+
+#endif
+