MediaLog instances are always bound to a specific player, so there's
no reason for having it be ref-counted other than to confuse folk
into thinking it can be reused.

There seems to be no reason why this was ref counted. There were a
couple questionable places, WebSourceBufferImpl and
WebMediaPlayerMSCompositor, but neither is really using the MediaLog
so I either removed it or replaced it with a dummy log.

The RendererFactories are a bit questionable too, but they are owned
by the WebMediaPlayerImpl so should be fine.

BUG=711818,712310
TEST=compiles, nothing crashes, existing tests still pass.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2815303006
Cr-Commit-Position: refs/heads/master@{#465753}