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 under NaCl to workaround crbug.com/538243 .

[kjellander_chromium, 2016/10/04 06:06:21]

Change to:
Always enable optimization for Release and NaCl builds (to workaround
crbug.com/538243).

[fbarchard1, 2016/10/04 18:23:45]

Done.

+  # enable O2 and ltcg.

[kjellander_chromium, 2016/10/04 06:06:21]

You might want to rephrase this to something like:
This enables O2 and ltcg on Windows.

[fbarchard1, 2016/10/04 18:23:45]

Done.
Its not just for Windows.  Arm release builds dramatically slower in Release
mode with -Os.

+  if (!is_debug || is_nacl) {
     configs -= [ "//build/config/compiler:default_optimization" ]
     configs += [ "//build/config/compiler:optimize_max" ]
   }
 }
 
 if (libyuv_use_neon) {
   static_library("libyuv_neon") {
     sources = [
       # ARM Source Files
       "source/compare_neon.cc",
@@ skipping 151 matching lines @@
   }
 
   executable("cpuid") {
     sources = [
       # sources
       "util/cpuid.c"
     ]
     deps = [ ":libyuv" ]
   }
 }