Reduce external dependencies on //device/vr

In preparation for creating a content/public interface method to get
VRDeviceProvider objects, this moves that interface type to
device/vr/public/cpp, and adds a build target for it (since it is a
class).
Additionally, removes a bunch of includes of vr_device.h which were
no longer necessary (and should only be necessary within device/vr).

Bug: 1070807
Change-Id: If9e08a495d654821838d2cc57047e4c3c7cdc933
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2155073
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760730}

• chrome/browser/BUILD.gn[diff]
• chrome/browser/android/vr/arcore_device/arcore_device_provider.h[diff]
• chrome/browser/vr/BUILD.gn[diff]
• chrome/browser/vr/service/browser_xr_runtime_impl.cc[diff]
• chrome/browser/vr/service/browser_xr_runtime_impl.h[diff]
• chrome/browser/vr/service/isolated_device_provider.h[diff]
• chrome/browser/vr/service/vr_service_impl.cc[diff]
• chrome/browser/vr/service/vr_service_impl.h[diff]
• chrome/browser/vr/service/xr_runtime_manager_impl.cc[diff]
• chrome/browser/vr/service/xr_runtime_manager_impl.h[diff]
• chrome/browser/vr/service/xr_runtime_manager_unittest.cc[diff]
• device/vr/BUILD.gn[diff]
• device/vr/android/arcore/arcore_device_provider_factory.cc[diff]
• device/vr/android/gvr/gvr_device_provider.h[diff]
• device/vr/orientation/orientation_device_provider.h[diff]
• device/vr/public/cpp/BUILD.gn[Added - diff]
• device/vr/public/cpp/vr_device_provider.cc[Added - diff]
• device/vr/public/cpp/vr_device_provider.h[Renamed from device/vr/vr_device_provider.h - diff]
• device/vr/test/fake_vr_device.h[diff]
• device/vr/test/fake_vr_device_provider.h[diff]
• device/vr/vr_device_base.cc[diff]