Introduce AudioSourceProviderClientLockScope.

It makes lock/unlock of AudioSourceProviderClient easier.
This class holds a strong reference to AudioSourceProviderClient. So we
don't need MediaElementAudioSourceNode::m_keepAliveWhileLocking.

BUG=395940

Review URL: https://codereview.chromium.org/416333002

git-svn-id: svn://svn.chromium.org/blink/trunk@178999 bbb929c8-8fbe-4397-9dbb-9b2b20218538