Use current time to calculate context disposal rate.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/679363003

Cr-Commit-Position: refs/heads/master@{#25212}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25212 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Remove heap size filter for context disposal garbage collection.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/712563002

Cr-Commit-Position: refs/heads/master@{#25213}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25213 ce2b1a6d-e550-0410-aec6-3dcde31c8c00