Add ability to use system-av1 (media-libs/libaom and media-libs/dav1d) instead of bundled.

--- a/config/external/moz.build
+++ b/config/external/moz.build
@@ -37,8 +37,9 @@ if not CONFIG['MOZ_SYSTEM_LIBVPX']:
     external_dirs += ['media/libvpx']
 
 if CONFIG['MOZ_AV1']:
-    external_dirs += ['media/libaom']
-    external_dirs += ['media/libdav1d']
+    if not CONFIG['MOZ_SYSTEM_AV1']:
+        external_dirs += ['media/libaom']
+        external_dirs += ['media/libdav1d']
 
 if not CONFIG['MOZ_SYSTEM_PNG']:
     external_dirs += ['media/libpng']
--- a/config/system-headers.mozbuild
+++ b/config/system-headers.mozbuild
@@ -1316,6 +1316,14 @@ if CONFIG['MOZ_ENABLE_CONTENTMANAGER']:
         'SelectSingleContentItemPage.h',
     ]
 
+if CONFIG['MOZ_SYSTEM_AV1']:
+    system_headers += [
+        'aom/aom_decoder.h',
+        'aom/aomdx.h',
+        'aom/aom_image.h',
+        'dav1d/dav1d.h',
+    ]
+
 if CONFIG['MOZ_SYSTEM_LIBVPX']:
     system_headers += [
         'vpx_mem/vpx_mem.h',
--- a/dom/media/platforms/moz.build
+++ b/dom/media/platforms/moz.build
@@ -78,6 +78,11 @@ if CONFIG['MOZ_AV1']:
         'agnostic/AOMDecoder.cpp',
         'agnostic/DAV1DDecoder.cpp',
     ]
+    if CONFIG['MOZ_SYSTEM_AV1']:
+        CXXFLAGS += CONFIG['MOZ_SYSTEM_LIBAOM_CFLAGS']
+        OS_LIBS += CONFIG['MOZ_SYSTEM_LIBAOM_LIBS']
+        CXXFLAGS += CONFIG['MOZ_SYSTEM_LIBDAV1D_CFLAGS']
+        OS_LIBS += CONFIG['MOZ_SYSTEM_LIBDAV1D_LIBS']
 
 if CONFIG['MOZ_OMX']:
     EXPORTS += [
--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -464,6 +464,24 @@ def av1(value, target, c_compiler):
 set_config('MOZ_AV1', av1)
 set_define('MOZ_AV1', av1)
 
+option('--with-system-av1',
+       help="Use system av1 (located with pkgconfig)")
+
+system_libaom_info = pkg_check_modules('MOZ_SYSTEM_LIBAOM', 'aom >= 1.0.0',
+                                       when='--with-system-av1')
+
+system_libdav1d_info = pkg_check_modules('MOZ_SYSTEM_LIBDAV1D', 'dav1d >= 0.1.1',
+                                         when='--with-system-av1')
+
+@depends(system_libaom_info, system_libdav1d_info)
+def system_av1(system_libaom_info, system_libdav1d_info):
+    has_av1_libs = False
+    if system_libaom_info and system_libdav1d_info:
+        has_av1_libs = True
+    return has_av1_libs
+
+set_config('MOZ_SYSTEM_AV1', depends_if(system_av1)(lambda _: True))
+
 # Built-in fragmented MP4 support.
 # ==============================================================
 option('--disable-fmp4', env='MOZ_FMP4',