Enable optimize max for arm in GN + update docs

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index fd8e231270110773c20d38c5a7ad3f4542fc41db..ca3ae832ae28fe52cfb50aae35aa73821ab62600 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -98,8 +98,9 @@ static_library("libyuv") {
     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" ]
   }