Bump MaxRegularSpaceAllocationSize to InitialSemiSpaceSize() * 4/5

to allow allocation of large packed arrays in paged spaces.

BUG=v8:2790
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23604023

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16475 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix ChangeLog entry for 3.20.15

BUG=v8:2845
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/23871002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16476 ce2b1a6d-e550-0410-aec6-3dcde31c8c00