third_party/libgav1: Build libgav1 on ChromeOS x86/x64

The Obu parser implementation in libgav1 will be used in hardware
accelerators on ChromeOS x86/x64. This CL builds libgav1
on ChromeOS x86/64. Thanks to the linker swiping unnecessary
objects, the chrome binary size increases by 43 kbytes
(= 191087912 - 191040232, 0.025% in ratio) with an av1 hardware
accelerator implementation (crrev.com/c/1921588).

Bug: 1029212
Test: Build chrome for chromeos x86
Change-Id: Ie9f0b3b551e3ea5138cc5ed85536498a9166da86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397596
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: James Zern <jzern@google.com>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817345}

third_party/libgav1/BUILD.gn

@@ -39,7 +39,7 @@ config("private_libgav1_config") {
   }
 }
 
-if (enable_libgav1_decoder) {
+if (enable_libgav1_decoder || use_libgav1_parser) {
   # Separate from libgav1 because utils/constants.cc and dsp/constants.cc
   # generate the same object file, constants.o.
   source_set("libgav1_utils") {
@@ -64,6 +64,7 @@ if (enable_libgav1_decoder) {
 
     sources = gav1_dsp_sources
   }
+
   static_library("libgav1") {
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]

third_party/libgav1/options.gni

@@ -8,4 +8,6 @@ declare_args() {
   # Enable libgav1 decoder.
   enable_libgav1_decoder =
       is_chromeos && (target_cpu == "arm" || target_cpu == "arm64")
+  use_libgav1_parser =
+      is_chromeos && (target_cpu == "x86" || target_cpu == "x64")
 }