Remove HTMLMediaElement::m_sendProgressEvents

It is always true, and it appears to have been that way since it was
added in 2009 (r40857).

BUG=none

Review URL: https://codereview.chromium.org/57473002

git-svn-id: svn://svn.chromium.org/blink/trunk@161245 bbb929c8-8fbe-4397-9dbb-9b2b20218538