ui::Cursor used to use ints across the codebase to identify
cursors. This patch changes that to an enum class, changes all usages of
these constants in chrome, and then creates an EnumTraits mapping of
this new enum to the mojo transport enum.

This means cursor enums are now validated when they previously were just
assumed to be correct.

BUG=705037
R=sky,tsepez
TBR=jam

Review-Url: https://codereview.chromium.org/2833163002
Cr-Commit-Position: refs/heads/master@{#467047}