[Messages] Init MessageDispatcherBridge::messages_enabled_for_embedder_

I discovered this while working on unittests related to
crbug.com/1308214. The fact that this boolean member variable isn't
initialized means that it takes an undefined value, which is problematic
(a) for embedders that don't call MessageDispatcherBridge::Initialize()
[such as WebLayer], and (b) for Chrome being able to reliably
distinguish between when MessageDispatcherBridge::Initialize() has not
yet been called from when it has been called.

We also add a test that fails without the fix (the boolean gets a
default value of true at least in my local build configuration).

Bug: 1309043, 1308214
Change-Id: I5714f1c066427bc3b85f7d62cc0ff8011530a7f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3542022
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#984236}