Add keying on backend to ANGLE Display caching. Webkit needs to use one ANGLE display with OpenGL and another with Metal. Our display caching is interfering with this, so it should be updated to also key on the selected backend. Bug: angleproject:6978 Change-Id: I9ec47a28e16646f5fc75956e3bd78236990cc392 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3440780 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>