Resolve jumbo conflict in video decoders (media/filters)

When building using jumbo, files gets merged. Files and
variables with the same name may merge into the same
namespace or scope and conflict. This happens in the files:
media/filters/aom_video_decoder.cc
media/filters/ffmpeg_video_decoder.cc
media/filters/vpx_video_decoder.cc

This commit solves the issue by renaming the function
GetThreadCount to something more file specific. It also
moves some constants to that function.

Bug: 867350
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Id57aa6cc6ac10acd0073eee032d0d04592ba87e4
Reviewed-on: https://chromium-review.googlesource.com/1151311
Commit-Queue: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham (In Paris) <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578593}
3 files changed