Expose use_webaudio_ffmpeg as build arg

Needed to disable use of ffmpeg for headless builds.

Change-Id: I9fc2eed035cd859f8111fd2068a529ddbbdbdfc8
Reviewed-on: https://chromium-review.googlesource.com/698504
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507482}

third_party/WebKit/Source/config.gni

@@ -20,6 +20,9 @@ declare_args() {
 
   # If true, defaults image interpolation to low quality.
   use_low_quality_image_interpolation = is_android
+
+  # If true, ffmpeg will be used for decoding audio.
+  use_webaudio_ffmpeg = !is_mac && !is_android
 }
 
 # Whether Android build uses OpenMAX DL FFT. Currently supported only on
@@ -30,8 +33,6 @@ use_openmax_dl_fft =
                    (current_cpu == "arm" && arm_version >= 7) ||
                    current_cpu == "arm64" || current_cpu == "mipsel")
 
-use_webaudio_ffmpeg = !is_mac && !is_android
-
 # TODO(tkent): Remove this flag after the great mv. crbug.com/760462
 snake_case_source_files = false