disable object-size santizer in debug builds

This avoids a warning-as-error:

 clang-5.0: error: the object size sanitizer has no effect at -O0, but
 is explicitly enabled:

Change-Id: I53a16acc3e743b42fe9c2f35919d3c09d5d601b7
Reviewed-on: https://skia-review.googlesource.com/55720
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index 1a61c28..dd28849 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -236,9 +236,12 @@
     # or pass one of the couple common aliases used by the bots.
     sanitizers = sanitize
     if (sanitize == "ASAN") {
-      sanitizers = "address,bool,integer-divide-by-zero,nonnull-attribute,null,object-size,return,returns-nonnull-attribute,shift,signed-integer-overflow,unreachable,vla-bound,vptr"
+      sanitizers = "address,bool,integer-divide-by-zero,nonnull-attribute,null,return,returns-nonnull-attribute,shift,signed-integer-overflow,unreachable,vla-bound,vptr"
       if (!is_mac) {
-        sanitizers += ",function"
+        sanitizers += ",function"  # Not supported on Mac.
+      }
+      if (!is_debug) {
+        sanitizers += ",object-size"  # No-op with somewhat annoying warning at -O0.
       }
     } else if (sanitize == "TSAN") {
       sanitizers = "thread"