Add support for configurable -OX optimization levels for debug builds

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.
 
+declare_args() {
+  # The optimization level to use for debug builds.
+  if (is_android) {
+    # On Android we kind of optimize some things that don't affect debugging
+    # much even when optimization is disabled to get the binary size down.
+    debug_optimization_level = "s"
+  } else {
+    debug_optimization_level = "2"
+  }
+}
+
 import("//build/config/android/config.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
   import("//build/config/mips.gni")
 }
 if (is_posix) {
   import("//build/config/gcc/gcc_version.gni")
 }
@@ skipping 721 matching lines @@
   }
   ldflags = common_optimize_on_ldflags
 }
 
 # Turn off optimizations.
 config("no_optimize") {
   if (is_win) {
     # The only difference on windows is that the inlining is less aggressive.
     # (We accept the default level). Otherwise it is very slow.
     cflags = [
-      "/O2",  # Do some optimizations.
+      "/O${debug_optimization_level}",  # Do some optimizations.
       "/Oy-",  # Disable omitting frame pointers, must be after /O2.
     ]
   } else if (is_android) {
     # On Android we kind of optimize some things that don't affect debugging
     # much even when optimization is disabled to get the binary size down.
     cflags = [
-      "-Os",
+      "-O${debug_optimization_level}",
       "-fdata-sections",
       "-ffunction-sections",
     ]
     ldflags = common_optimize_on_ldflags
   } else {
     cflags = [
-      "-O2",
+      "-O${debug_optimization_level}",
       "-fdata-sections",
       "-ffunction-sections",
     ]
   }
 }
 
 # Symbols ----------------------------------------------------------------------
 
 config("symbols") {
   if (is_win) {
     import("//build/toolchain/goma.gni")
     if (use_goma) {
       cflags = [ "/Z7" ]  # No PDB file
     } else {
       cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
     }
     ldflags = [ "/DEBUG" ]
   } else {
     cflags = [
       "-g3",
       "-ggdb3",
     ]
   }
 }