Fix integer overflow

N.B. libvpx will later check that frame diminsions don't exceed
codec specific limits.

Bug: 1127590
Change-Id: I20d635169a63e81eed40a7cc3ee4a77284a753ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411672Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807138}

media/video/vpx_video_encoder.cc

@@ -65,7 +65,7 @@ Status SetUpVpxConfig(const VideoEncoder::Options& opts,
     config->rc_target_bitrate = opts.bitrate.value() / 1000;
   } else {
     config->rc_end_usage = VPX_VBR;
-    config->rc_target_bitrate = static_cast<double>(opts.width * opts.height) /
+    config->rc_target_bitrate = double{opts.width} * double{opts.height} /
                                 config->g_w / config->g_h *
                                 config->rc_target_bitrate;
   }