Simplify allocation tracker API

Deprecated separate methods for starting/stopping allocation tracking in favor of a bool param to Start/StopTrackingHeapObjects.

BUG=None
LOG=N
R=loislo@chromium.org, mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/96933003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18197 ce2b1a6d-e550-0410-aec6-3dcde31c8c00