There are some unused bits in a BigInt's bit field. We never read their
their values explicitly, but whenever the entire object is moved around
(for serialization, or GC), this uninitialized memory is accessed. This
patch fixes that by initializing the entire field after allocation of a
BigInt, not just the bits we actually use.

Bug: chromium:818109
Change-Id: I5a4d24c3240242157b902c696fa9bb779799280d
Reviewed-on: https://chromium-review.googlesource.com/946676Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51708}