Apply OwnPtr|PassOwnPtr to member variables and arguments in ContextFeatures.

WebViewImpl has used m_featureSwitchClient unnecessarily in order to pass it to provideContextFeaturesTo().
To remove it, we need to apply OwnPtr|PassOwnPtr to member variable and parameter types.

Additionally m_midiClientProxy should be removed because it wasn't removed in r170746.

BUG=N/A

Review URL: https://codereview.chromium.org/224113006

git-svn-id: svn://svn.chromium.org/blink/trunk@170877 bbb929c8-8fbe-4397-9dbb-9b2b20218538