Un-flake test-cpu-profiler/SampleWhenFrameIsNotSetup

It is OK for FindChild to return NULL. If the child must
exist GetChild should be used to force the assertion.

BUG=v8:2628
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/15786004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14809 ce2b1a6d-e550-0410-aec6-3dcde31c8c00