Use actual incremental marking throughput in IdleNotification to estimate marking step size.

BUG=
R=jochen@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/465473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23224 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix arm64 build.

TBR=hpayer@chromium.org

Review URL: https://codereview.chromium.org/490643006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23225 ce2b1a6d-e550-0410-aec6-3dcde31c8c00