Write NULL as "" so readString() always returns a non-NULL string.

BUG=
R=reed@google.com

Review URL: https://codereview.chromium.org/22862002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@10754 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkWriter32.h b/core/SkWriter32.h
index 82cf346..308e288 100644
--- a/core/SkWriter32.h
+++ b/core/SkWriter32.h
@@ -168,7 +168,9 @@
      *  Writes a string to the writer, which can be retrieved with
      *  SkReader32::readString().
      *  The length can be specified, or if -1 is passed, it will be computed by
-     *  calling strlen(). The length must be < 0xFFFF
+     *  calling strlen(). The length must be < max size_t.
+     *
+     *  If you write NULL, it will be read as "".
      */
     void writeString(const char* str, size_t len = (size_t)-1);