Include m_attachedElement in MediaSourceBase::hasPendingActivity() condition.

m_attachedElement needs to be added to hasPendingActivity() so that a reference
is held the entire time it is considered "attached" to an HTMLMediaElement. This
includes the brief period it is attached, but not open yet.

BUG=330026

Review URL: https://codereview.chromium.org/112373005

git-svn-id: svn://svn.chromium.org/blink/trunk@164245 bbb929c8-8fbe-4397-9dbb-9b2b20218538