When compiling against an OSX 10.7+ SDK, explicitly redeclare methods only available from an OSX 10.7+ SDK. This suppresses the clang warning -Wpartial-availability, which will be turned on in the future. BUG=chromium:471823 R=jiayl@webrtc.org, mark@chromium.org Review URL: https://webrtc-codereview.appspot.com/44359004 Cr-Commit-Position: refs/heads/master@{#9163}