Add a dedicated "optimize_speed" config to GN.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 0cba95ff63396ac21889680191dd8777050cce26..57a4a3c2853ede9c9a9c97e9e31f5cd255ae0e45 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1399,6 +1399,17 @@ config("optimize_max") {
   }
 }
 
+# This config can be used to override the default settings for per-component
+# and whole-program optimization, optimizing the particular target for speed
+# instead of code size. This should be used carefully by people who know
+# exactly what they want; nearly every component should use "optimize_max"
+# instead.
+# TODO(crbug.com/621335) - rework how all of these configs are related
+# so that we don't need this disclaimer.
+config("optimize_posix_target_for_speed") {
+  cflags = [ "-O3" ] + common_optimize_on_cflags
+}
+
 # The default optimization applied to all targets. This will be equivalent to
 # either "optimize" or "no_optimize", depending on the build flags.
 config("default_optimization") {