Move vr features.gni into its own directory.

This fixes some weird issues we were encountering, where we had to hide some build targets behind (!is_ios) after adding our features target to ui/gfx.
I'm not sure if any extra code was being compiled in, but asserts in gn files were being hit related to things that shouldn't be included on ios.
It seems other features have already done this sort of thing.

TBR=brettw@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2840563004
Cr-Commit-Position: refs/heads/master@{#467113}
Committed: https://chromium.googlesource.com/chromium/src/+/faae24707ba8852ea92b56bcf77aa19878aacdab
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2840563004
Cr-Commit-Position: refs/heads/master@{#467167}
19 files changed