Enable optimize max for arm in GN + update docs

BUILD.gn

 # Copyright 2014 The LibYuv Project Authors. All rights reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS. All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("libyuv.gni")
 import("//testing/test.gni")
@@ skipping 80 matching lines @@
   }
 
   if (libyuv_use_neon) {
     deps += [ ":libyuv_neon" ]
   }
 
   if (libyuv_use_msa) {
     deps += [ ":libyuv_msa" ]
   }
 
-  if (is_nacl) {
-    # Always enable optimization under NaCl to workaround crbug.com/538243 .
+  # Always enable optimization for Release and NaCl builds (to workaround
+  # crbug.com/538243).
+  if (!is_debug || is_nacl) {
     configs -= [ "//build/config/compiler:default_optimization" ]
+    # Enable optimize for speed (-O2) over size (-Os).
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 }
 
 if (libyuv_use_neon) {
   static_library("libyuv_neon") {
     sources = [
       # ARM Source Files
       "source/compare_neon.cc",
       "source/compare_neon64.cc",
@@ skipping 150 matching lines @@
   }
 
   executable("cpuid") {
     sources = [
       # sources
       "util/cpuid.c"
     ]
     deps = [ ":libyuv" ]
   }
 }