[vr] Update assets to version 2.3.

This darkens the normal gradient.

Bug: 842199
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ic22ecf9262fc9641ca766ab14802c90804d5474d
Reviewed-on: https://chromium-review.googlesource.com/1055651Reviewed-by: Biao She <bshe@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557918}

chrome/browser/resources/vr/assets/VERSION

 MAJOR=2
-MINOR=2
\ No newline at end of file
+MINOR=3
\ No newline at end of file

chrome/browser/resources/vr/assets/google_chrome/normal_gradient.png.sha1

-bbe1145ae7778d7d50b02c54fd1bf4b62bbc04db
\ No newline at end of file
+446510d78dc522fcef45cb88a7a7c4c6d72e17ed
\ No newline at end of file