Merged r15489, r15583 into 3.19 branch.

MIPS: Fix bug in StringConstructCode builtin.

MIPS: Adjust default Heap sizes.

R=jkummerow@chromium.org
BUG=

Review URL: https://codereview.chromium.org/19197002

git-svn-id: https://v8.googlecode.com/svn/branches/3.19@15671 ce2b1a6d-e550-0410-aec6-3dcde31c8c00