GN: release -> -O3, to match GYP

-O3 does trigger some important compiler optimizations that -Os does not,
like autovectorization.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2288813002

Review-Url: https://codereview.chromium.org/2288813002

gn/BUILD.gn

@@ -105,7 +105,7 @@ config("default") {
 }
 
 config("release") {
-  cflags = [ "-Os" ]
+  cflags = [ "-O3" ]
   defines = [ "NDEBUG" ]
 }