[Fuchsia] Reduce media buffer sizes for Fuchsia

By default media demuxer buffers 150MB of video and 12MB of audio. This
may be to much for some Fuchsia devices. Use low values of 30MB and 2MB.

Bug: ZX-4768
Change-Id: I084f85dcc92fa54c56fd06ab8270f4fb73762410
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725240
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682417}

media/base/BUILD.gn

@@ -383,7 +383,7 @@ jumbo_source_set("base") {
     public_deps += [ "//media/base/win:d3d11" ]
   }
 
-  if (is_chromecast) {
+  if (is_chromecast || is_fuchsia) {
     sources += [ "demuxer_memory_limit_low.cc" ]
   } else if (is_android) {
     sources += [ "demuxer_memory_limit_android.cc" ]