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..3cf72220237662772be19973c54a9c9de54012fd 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -80,6 +80,12 @@ 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.
+  # Linux & Mac favor speed over size.
+  # TODO(brettw) it's weird that Mac and desktop Linux are different. We should
+  # explore favoring size over speed in this case as well.
+  optimize_for_size = is_android || is_ios
 }
 
 # Apply the default logic for these values if they were not set explicitly.
@@ -1209,13 +1215,10 @@ 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 {
-    # Linux & Mac favor speed over size.
-    # TODO(brettw) it's weird that Mac and desktop Linux are different. We should
-    # explore favoring size over speed in this case as well.
     cflags = [ "-O2" ] + common_optimize_on_cflags
   }
   ldflags = common_optimize_on_ldflags