[gn] Experiment: Reset -O3 to -O2.

Temporary commit to see performance data. Will be reverted
shortly after.

BUG=chromium:622332
TBR=vogelheim, jochen
NOTRY=true

Review-Url: https://codereview.chromium.org/2135313002
Cr-Commit-Position: refs/heads/master@{#37667}

gni/v8.gni

@@ -57,7 +57,7 @@ if (is_debug && !v8_optimized_debug) {
   # TODO(crbug.com/621335) Rework this so that we don't have the confusion
   # between "optimize_speed" and "optimize_max".
   if (is_posix && !is_android && !is_nacl && !using_sanitizer) {
-    add_configs += [ "//build/config/compiler:optimize_speed" ]
+    add_configs += [ "//build/config/compiler:optimize_max" ]
   } else {
     add_configs += [ "//build/config/compiler:optimize_max" ]
   }