Add GN arg to optimize for size

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 9583f779dfff6f1ae3a7c14f00a5c384789b2852..15957695cce3e249650b4c37aefaba63aa6c1c64 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -80,6 +80,9 @@ declare_args() {
   # If true, gold linker will save symbol table inside object files.
   # This speeds up gdb startup by 60%
   gdb_index = false
+
+  # If true, optimize for size. Does not affect windows builds.
+  optimize_for_size = is_android || is_ios
 }
 
 # Apply the default logic for these values if they were not set explicitly.
@@ -1209,7 +1212,7 @@ config("optimize") {
     # Favor size over speed, /O1 must be before the common flags. The GYP
     # build also specifies /Os and /GF but these are implied by /O1.
     cflags = [ "/O1" ] + common_optimize_on_cflags + [ "/Oi" ]
-  } else if (is_android || is_ios) {
+  } else if (optimize_for_size) {
     # Favor size over speed.
     cflags = [ "-Os" ] + common_optimize_on_cflags
   } else {