Add GN arg to optimize for size

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 62 matching lines @@
     msvs_xtree_patched = false
   }
 
   # Omit unwind support in official builds to save space.
   # We can use breakpad for these builds.
   exclude_unwind_tables = is_chrome_branded && is_official_build
 
   # 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.
 if (gold_path == false) {
   if (use_gold) {
     gold_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
                             root_build_dir)
   } else {
     gold_path = ""
   }
@@ skipping 1109 matching lines @@
     }
   }
 }
 
 # Default "optimization on" config.
 config("optimize") {
   if (is_win) {
     # 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.

[slan, 2016/04/12 20:50:19]

nit: It might make more sense now to move this comment up to the flag
declaration (where you've assigned the default value based on the platform)

[bcf, 2016/04/12 20:54:38]

Done.

     # 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
 }
 
 # Turn off optimizations.
 config("no_optimize") {
   if (is_win) {
@@ skipping 170 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }