Fix Heap::AllocateFillerObject() to use double alignment param.

This is called from Runtime, and was dropping the requested alignment
for doubles. Needed for upcoming MIPS CL to remove some more
workarounds for load/store doubles.

BUG=

Review URL: https://codereview.chromium.org/1131123003

Cr-Commit-Position: refs/heads/master@{#28413}