Make WebMediaPlayer::didLoadingProgress() non-const.

The Chromium-side change is https://codereview.chromium.org/275303002, which must be committed first.

BUG=360251

Review URL: https://codereview.chromium.org/272223004

git-svn-id: svn://svn.chromium.org/blink/trunk@173980 bbb929c8-8fbe-4397-9dbb-9b2b20218538