Cherry-pick 2213bac. rdar://116372865

    Cherry-pick https://chromium.googlesource.com/webm/libvpx.git/+/51057f4ba894e13f9bba278905bacf6aaaecd992
    https://bugs.webkit.org/show_bug.cgi?id=262365
    rdar://116233358

    Reviewed by Mark Lam.

    * Source/ThirdParty/libwebrtc/Source/third_party/libvpx/source/libvpx/vp8/encoder/onyx_if.c:
    (vp8_change_config):

    Canonical link: https://commits.webkit.org/267815.127@safari-7617-branch

Canonical link: https://commits.webkit.org/265870.609@safari-7616.2.9.10-branch

Source/ThirdParty/libwebrtc/Source/third_party/libvpx/source/libvpx/vp8/encoder/onyx_if.c

@@ -1445,6 +1445,11 @@ void vp8_change_config(VP8_COMP *cpi, VP8_CONFIG *oxcf) {
   last_h = cpi->oxcf.Height;
   prev_number_of_layers = cpi->oxcf.number_of_layers;
 
+  if (cpi->initial_width) {
+    // TODO(https://crbug.com/1486441): Allow changing thread counts; the
+    // allocation is done once in vp8_create_compressor().
+    oxcf->multi_threaded = cpi->oxcf.multi_threaded;
+  }
   cpi->oxcf = *oxcf;
 
   switch (cpi->oxcf.Mode) {