add safemath version of readbuffer->skip

Bug: skia:
Change-Id: I6a5d24569cc1bd93bd190d81e3375993da9c56f0
Reviewed-on: https://skia-review.googlesource.com/88581
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/src/core/SkSafeMath.h b/src/core/SkSafeMath.h
index 1005d06..7afd482 100644
--- a/src/core/SkSafeMath.h
+++ b/src/core/SkSafeMath.h
@@ -48,6 +48,10 @@
         return add(x, alignment - 1) & ~(alignment - 1);
     }
 
+    // These saturate to their results
+    static size_t Add(size_t x, size_t y);
+    static size_t Mul(size_t x, size_t y);
+
 private:
     uint32_t mul32(uint32_t x, uint32_t y) {
         uint64_t bx = x;