Converts ui::Accelerator::type to an enum

Accelerator::type_ was an EventType, but this is overkill as the type
can only be pressed or released. So, this converts the type to an enum
with those two values.

I'm doing this as I'm going to write a mojom for Accelerator and it's
better to restrict the mojom to the actual values.

BUG=701815
TEST=covered by tests
R=msw@chromium.org

Review-Url: https://codereview.chromium.org/2751323002
Cr-Commit-Position: refs/heads/master@{#457492}
9 files changed