libgav1,BUILD.gn: force optimize, arm flags

-O2 is ~11-17% faster than -Os on a pixel 3a using a ndk build of the
decoder (r21d)

Bug: 1047051
Change-Id: Ifa67bcd7352f535a71f726c32320d6c0353263d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340497Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#796468}

third_party/libgav1/BUILD.gn

@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/arm.gni")
 import("//third_party/libgav1/libgav1_srcs.gni")
 import("//third_party/libgav1/options.gni")
 
@@ -23,12 +24,28 @@ config("public_libgav1_config") {
   ]
 }
 
+config("private_libgav1_config") {
+  configs = []
+
+  # dsp intrinsics will generate much better code when optimized for speed
+  # rather than size.
+  if (!is_debug) {
+    configs += [ "//build/config/compiler:optimize_max" ]
+  }
+  if (current_cpu == "arm64" ||
+      (current_cpu == "arm" && arm_version >= 7 && arm_use_neon)) {
+    # The default thumb mode will impact performance of dsp intrinsics.
+    configs += [ "//build/config/compiler:compiler_arm" ]
+  }
+}
+
 if (enable_libgav1_decoder) {
   # Separate from libgav1 because utils/constants.cc and dsp/constants.cc
   # generate the same object file, constants.o.
   source_set("libgav1_utils") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
+    configs += [ ":private_libgav1_config" ]
 
     public_configs = [ ":public_libgav1_config" ]
 
@@ -40,6 +57,7 @@ if (enable_libgav1_decoder) {
   source_set("libgav1_dsp") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
+    configs += [ ":private_libgav1_config" ]
 
     deps = [ ":libgav1_utils" ]
     public_configs = [ ":public_libgav1_config" ]
@@ -49,6 +67,7 @@ if (enable_libgav1_decoder) {
   static_library("libgav1") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
+    configs += [ ":private_libgav1_config" ]
 
     public_configs = [ ":public_libgav1_config" ]
     public_deps = [