GitLab

Sync our IDL file with https://github.com/w3c/gamepad/pull/62 ("Switch from
WebIDL arrays to FrozenArray<>s in the IDLs").

WebIDL has not had array types since 2015, so finally make our IDL files
compliant with modern WebIDL following the spec fix.

It is important to note that this change modifies the existing behavior
slightly.
- |axes| and |buttons| are now frozen objects with all the related
  consequences for its properties and prototype.
- Those two attributes now return the same _object_ until their values
  change instead of always returning a new object on access.

Doing so aligns our code with both the spec as well as Gecko, which has done
the above ever since it implemented the Gamepad spec.

Bug: 740875
Change-Id: Ifb618c9d4f8860eb55efc882e701dae7390808a5
Reviewed-on: https://chromium-review.googlesource.com/595979Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#491440}