<!-- Please describe your changes on the following line: -->
Gamepad API implementation. Tested with HTC Vive and Daydream controllers ;)
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#10977
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because : current gamepad tests are manual (https://github.com/w3c/web-platform-tests/tree/master/gamepad). There is a open issue about the best way to test WebVR/Gamepad without real devices https://github.com/w3c/webvr/issues/187. We'll work on the testing suite & mock devices/data on a separate issue.
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Source-Repo: https://github.com/servo/servo
Source-Revision: 26c45275ffaccf746e47606a74b3aee519673e54
--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 5ae2853a5fc26b6a0fedc0cbb9b3eef9e6ad6049
975fde609c
