Add the rtc_build_openmax_dl variable to the GN build.

For symmetry with the gyp build.

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/49109005

Cr-Commit-Position: refs/heads/master@{#9083}

webrtc/build/webrtc.gni

@@ -44,6 +44,7 @@ declare_args() {
   rtc_build_vp9 = true
   rtc_build_ssl = true
   rtc_build_opus = true
+  rtc_build_openmax_dl = true
 
   # Disable by default.
   rtc_have_dbus_glib = false

webrtc/common_audio/BUILD.gn

@@ -116,7 +116,9 @@ source_set("common_audio") {
       "real_fourier_openmax.h",
     ]
     defines += [ "RTC_USE_OPENMAX_DL" ]
-    deps += [ "//third_party/openmax_dl/dl" ]
+    if (rtc_build_openmax_dl) {
+      deps += [ "//third_party/openmax_dl/dl" ]
+    }
   }
 
   if (current_cpu == "arm") {