Fix a crash in WindowCapturereMac when capture() fails.
Fix a crash in WindowCapturereMac when capture() fails.

BUG=http://code.google.com/p/chromium/issues/detail?id=359985
R=sergeyu@chromium.org

Review URL: https://webrtc-codereview.appspot.com/11219004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f040bd8fa3c0014ab32dad4787f7b96994fa9fda

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc