Fixes 2D-to-WebVR site transitions when browsing in VR.

- Renames GetVRDevice(...) to CreateVRDisplayInfo(...) for better understandability.
- Refactores the addition for services in VRDeviceManager to simplify the code.
- Makes the client of VRDisplayImpl and VRServiceImpl private. Added delegate methods where calls to the client are necessary.
- Makes sure that VRDisplayImpl's client is initialized in the constructor. A call to an uninitialized client was the actual source of the bug.
- Moves the instantiation of VRDisplayImpl objects to VRServiceImpl since VRServiceImpl is the owner of the VRDisplayImpls.
- Adds class comments to the touched classes.
- Adaptes the unit tests accordingly.

BUG=703318

Review-Url: https://codereview.chromium.org/2746233002
Cr-Commit-Position: refs/heads/master@{#458483}
15 files changed