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In additon to moving those declaration places, changes the type of
kMinSupportedNumConcurrentDecoders to const size_t from enum.

BUG=chromium:834170
TEST=VDA unittest on eve and kevin
TEST=VDA unittest on non-Chrome OS platforms in CQ.

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: I5b343adb64811f8f6dde2699af60542c666890af
Reviewed-on: https://chromium-review.googlesource.com/1027333
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Pawel Osciak <posciak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555265}