The WebAssembly spec defines that the name of a custom section can cause
a validation error. The streaming decoder, however, used a separate
Decoder object to decode the name, and thereby avoided a validation
error. With this CL the streaming decoder uses the main decoder to
decode the name of the custom section.

In addition this CL removes the test mjsunit/regress/wasm/regress-789952.
This test defined an invalid WebAssembly module and expected it to
compile. As it is a regression test, it makes no sense to fix the test.
The module is invalid because it defines the length of the custom section
to be '0', so there are no bytes in the custom section for its name.

R=clemensb@chromium.org
CC=thibaudm@chromium.org

Bug: v8:10126
Change-Id: I8cfc77c9a5916570d5362d5922e0179a29774da8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2041446
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66348}