HeapNumbers and MutableHeapNumber requires alignment for their double
value field but for now this field can be misaligned on 32-bit platforms.

According to code in Heap::GetFillToAlign() function, kDoubleUnaligned
doesn't actually mean "unaligned" but "aligned to half of double".

This CL fixes this misalignment.

Change-Id: I9b9c58d580bb287e7dad44bc96cd6b4593707b5e
Reviewed-on: https://chromium-review.googlesource.com/c/1470113
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59599}