Sync the DeviceOrientation and DeviceMotion interfaces with the spec

http://w3c.github.io/deviceorientation/spec-source-orientation.html

Ther are no changes to the generated code, as a nullable readonly
attribute is handled by bindings just like a non-nullable one.

BUG=460722
R=timvolodine@chromium.org

Review URL: https://codereview.chromium.org/1387453002

Cr-Commit-Position: refs/heads/master@{#353253}
4 files changed