Make WebMediaPlayer::LoadType UMA friendly.
Make WebMediaPlayer::LoadType UMA friendly.

See the chromium side change at https://codereview.chromium.org/1273973002/

BUG=517239

Review URL: https://codereview.chromium.org/1272423002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200083 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk