media/filter: Refine the thread number for vp9 decoding

This CL refined the decoding thread number for vp9.
the thread number before and after this CL:

resolution     before     after
640x360        2          2
1280x720       4          4
1920x1080      4          4
2048x1080      8          4
2560x1440      8          8
3840x2160      8          16
4096x2160      16         16

Bug: None
Change-Id: I1a8d76b518dc27152fd52b0e807b666f924f077c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2332338Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793745}

media/filters/vpx_video_decoder.cc

@@ -44,11 +44,11 @@ static int GetVpxVideoDecoderThreadCount(const VideoDecoderConfig& config) {
   // maximum number of tiles possible for higher resolution streams.
   if (config.codec() == kCodecVP9) {
     const int width = config.coded_size().width();
-    if (width >= 4096)
+    if (width >= 3840)
       desired_threads = 16;
-    else if (width >= 2048)
+    else if (width >= 2560)
       desired_threads = 8;
-    else if (width >= 1024)
+    else if (width >= 1280)
       desired_threads = 4;
   }