[views-mus] Support custom cursors.

This changes the mus interface to pass a CursorData instead of a
CursorType. The previous patch had mus clients create CursorDatas on
ozone platforms, and this patch passes CursorDatas across the mojo pipe
and passes it to the native ozone platform on the window server.

BUG=705037

Review-Url: https://codereview.chromium.org/2830703003
Cr-Commit-Position: refs/heads/master@{#467832}
49 files changed