Convert SkPicture's generation ID to a unique ID

This CL addresses linger code review comments on r14037 (Add generation ID to SkPicture https://codereview.chromium.org/222683002/)

R=reed@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/225283014

git-svn-id: http://skia.googlecode.com/svn/trunk@14079 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/core/SkTypes.h b/include/core/SkTypes.h
index 9b37604..74ddf02 100644
--- a/include/core/SkTypes.h
+++ b/include/core/SkTypes.h
@@ -315,6 +315,10 @@
 */
 #define SkMSec_LE(a, b)     ((int32_t)(a) - (int32_t)(b) <= 0)
 
+/** The generation IDs in Skia reserve 0 has an invalid marker.
+ */
+#define SK_InvalidGenID     0
+
 /****************************************************************************
     The rest of these only build with C++
 */