The bulk-memory proposal adds a new DataCount section that declares the
number of data segments that are expected to be seen in the Data
section. This is similar to the way the number of functions is split
between the Function and Code sections.

The DataCount section occurs before the Code section, so we can do
single-pass validation of the new `memory.init` and `memory.drop`
instructions, which have data segment indices as immediates.

Bug: v8:7747
Change-Id: Ibc5a7ee9336dbc5d0fd667572c42cb065c048e00
Reviewed-on: https://chromium-review.googlesource.com/c/1352792
Commit-Queue: Ben Smith <binji@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57951}