Update Display to allow for changing implementation objects. Previously it was possible to change from a D3D9 to D3D11 display because both were implemented with a DisplayD3D. Now that there is a DisplayGL, the DisplayImpl must be updated while maintaining the same EGLDisplay value. Allow the value of Display::mImplementation to be updated when the display is not already initialized. BUG=angle:890 Change-Id: Ic18a8a120218cd130a71b9aa044b6ec00006d6a0 Reviewed-on: https://chromium-review.googlesource.com/252250 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Nicolas Capens <capn@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>