Simplify MediaController with C++11 range-base for loops

Some uses of MediaElementSequence::const_iterator remain, where the
first media element in m_mediaElements is handled differently.

Review URL: https://codereview.chromium.org/612873002

git-svn-id: svn://svn.chromium.org/blink/trunk@182845 bbb929c8-8fbe-4397-9dbb-9b2b20218538