Moves alignment fill calculations into two static Heap methods.
Adds a Heap method to handle the complex case where filler is potentially needed before and after a heap object.
Makes DoubleAlignForDeserialization explicitly fill after an already
aligned object.

LOG=N
BUG=v8:4124

Review URL: https://codereview.chromium.org/1150593003

Cr-Commit-Position: refs/heads/master@{#28687}