Ensure double alignment when deserializing.

Ensure double alignment when deserializing.

R=rmcilroy@chromium.org
BUG=chromium:436510
LOG=N
TEST=compile with V8_OOL_CONSTANT_POOL=1, run d8 with --verify-heap --test

Review URL: https://codereview.chromium.org/759823006

Cr-Commit-Position: refs/heads/master@{#25534}