Hack together MSAN build.

src/core/SkMSAN.h

Index: src/core/SkMSAN.h
diff --git a/src/core/SkMSAN.h b/src/core/SkMSAN.h
index 7e544778d7655cbb2ee6de19565054c7a4cae579..1f32e5356c6f34d9b852635d8f0258eaeea7cbd8 100644
--- a/src/core/SkMSAN.h
+++ b/src/core/SkMSAN.h
@@ -8,11 +8,12 @@
 #ifndef SkMSAN_DEFINED
 #define SkMSAN_DEFINED
 
-#include <stddef.h>  // size_t
+#include "SkTypes.h"
 
 // Typically declared in LLVM's msan_interface.h.  Easier for us to just re-declare.
 extern "C" {
     void __msan_check_mem_is_initialized(const volatile void*, size_t);
+    void __msan_unpoison                (const volatile void*, size_t);
 }
 
 // Code that requires initialized inputs can call this to make it clear that
@@ -25,4 +26,15 @@ static inline void sk_msan_assert_initialized(const void* begin, const void* end
 #endif
 }
 
+// Lie to MSAN that this range of memory is initialized.
+// This can hide serious problems if overused.  Every use of this should refer to a bug.
+static inline void sk_msan_mark_initialized(const void* begin, const void* end, const char* skbug) {
+    SkASSERT(skbug && 0 != strcmp(skbug, ""));
+#if defined(__has_feature)
+    #if __has_feature(memory_sanitizer)
+        __msan_unpoison(begin, (const char*)end - (const char*)begin);
+    #endif
+#endif
+}
+
 #endif//SkMSAN_DEFINED