Update Skia milestone to 53 after 52 branch

TBR=reed@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1993203002

Review-Url: https://codereview.chromium.org/1993203002
diff --git a/include/core/SkMilestone.h b/include/core/SkMilestone.h
index f354aaf..f9072a0 100644
--- a/include/core/SkMilestone.h
+++ b/include/core/SkMilestone.h
@@ -5,5 +5,5 @@
  * found in the LICENSE file.
  */
 #ifndef SK_MILESTONE
-#define SK_MILESTONE 52
+#define SK_MILESTONE 53
 #endif