MIPS: Adjust default Heap sizes.

This change decreases the default Heap sizes on MIPS (sets the
same sizes as on Android).

Most MIPS hw has smaller RAM size than 700MB, which can lead
to test failures, if a test does not set the  Heap sizes explicitly
(like cctest/test-mark-compact/MarkCompactCollector).

BUG=

Review URL: https://codereview.chromium.org/18447003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15583 ce2b1a6d-e550-0410-aec6-3dcde31c8c00