Stop preloading vr module to avoid racey crash

It was possible for the vr split to be loaded in the background thread
before the application ClassLoader had been replaced with the chrome
ClassLoader. This was causing issues in the BundleUtils logic which
makes sure ClassLoaders have the correct parent. For now, we will just
disable vr preloading entirely since it is safer for merging to stable,
but we should be able to preload safely after the application
ClassLoader has been replaced.

Bug: 1184300
Change-Id: I87461e1093d1acbd47c11a7dfed8e44598a9234e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2733370


Commit-Queue: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Clark DuVall <cduvall@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859594}