add default impl for context methods on shader

src/core/SkShader.cpp

Index: src/core/SkShader.cpp
diff --git a/src/core/SkShader.cpp b/src/core/SkShader.cpp
index 4ddd29150b182f13505597e4449f3a046eeb9427..a449d0fd9d4e7835c686d25c76cd5b0960846696 100644
--- a/src/core/SkShader.cpp
+++ b/src/core/SkShader.cpp
@@ -63,6 +63,15 @@ bool SkShader::validContext(const SkBitmap& device,
     return this->computeTotalInverse(matrix, totalInverse);
 }
 
+SkShader::Context* SkShader::createContext(const SkBitmap&, const SkPaint&, const SkMatrix&,
+                                           void* storage) const {
+    return NULL;
+}
+
+size_t SkShader::contextSize() const {
+    return 0;

[scroggo, 2014/04/30 21:38:23]

This may assert in skia/ext/pixel_ref_utils_unittest.cc after
https://codereview.chromium.org/264463003/

[Dominik Grewe, 2014/05/01 09:18:43]

Yes, this will fail if a subclass doesn't overwrite this method. In [1] we'd
pass 0 as the allocation size to SkSmallAllocator which requires that the
allocation size is at least the size of T (= SkShader::Context).
Could we return 'sizeof(SkShader::Context)' instead? Otherwise we'd have to
check for 'contextSize() == 0' before trying to allocate.

[1]
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/s...

+}
+
 SkShader::Context::Context(const SkShader& shader, const SkBitmap& device,
                            const SkPaint& paint, const SkMatrix& matrix)
     : fShader(shader)