This CL separates the BigInt allocation code in the DataView BigInt
getters from the logic of the getters themselves.

This makes the code much easier to read and understand.

Change-Id: I9f7ee3fb819f0606dc631bac89e386f6fec43655
Reviewed-on: https://chromium-review.googlesource.com/1107632
Commit-Queue: Théotime Grohens <theotime@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54036}