Perform context disposal garbage collections only if contexts disposals happen at a high rate.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/710603003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25208 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
[arm64] Bug fix in sapefoint recording.

cp is handled by the register allocator and doesn't necessarily hold the
context pointer on arm64.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/695323005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25209 ce2b1a6d-e550-0410-aec6-3dcde31c8c00