[webcodecs] Set VEA default bitrate based on image size

As mentioned, this CL set the default bitrate based on image size
instead of setting a fixed value.

Bug: None
Change-Id: I694f71f5fef64165ef15f4f8d07012485297e2e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2389261
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805025}

media/video/video_encode_accelerator_adapter.cc

@@ -20,6 +20,11 @@
 namespace media {
 
 namespace {
+
+// HW encoders expect a nonzero bitrate, so |kVEADefaultBitratePerPixel| is used
+// to estimate bits per second for ~30 fps with ~1/16 compression rate.
+constexpr int kVEADefaultBitratePerPixel = 2;
+
 Status SetUpVeaConfig(VideoCodecProfile profile,
                       const VideoEncoder::Options& opts,
                       VideoEncodeAccelerator::Config* config) {
@@ -29,9 +34,11 @@ Status SetUpVeaConfig(VideoCodecProfile profile,
 
   *config = VideoEncodeAccelerator::Config(
       PIXEL_FORMAT_I420, gfx::Size(opts.width, opts.height), profile,
-      opts.bitrate.value_or(1000000));
+      opts.bitrate.value_or(opts.width * opts.height *
+                            kVEADefaultBitratePerPixel));
   return Status();
 }
+
 }  // namespace
 
 class VideoEncodeAcceleratorAdapter::SharedMemoryPool