Fix ASSERT in Sky when dragging outside the window on linux

Fix ASSERT in Sky when dragging outside the window on linux

We don't seem to receive mojo::EVENT_TYPE_MOUSE_EXITED when the mouse exits our
mojo::View. We should find out if that's a bug with the Mojo event system, but
for now this CL adds a fake cancel event the next time the mouse goes down to
at least keep the state machine on the right track.

BUG=https://github.com/domokit/mojo/issues/40
R=eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/4393ecb1208f1fbc9aed551b39eaaa2c31dcd18a

[eseidel, 2015-01-26 18:51:25]

eseidel@chromium.org changed reviewers:
+ eseidel@chromium.org

[eseidel, 2015-01-26 18:51:25]

lgtm

[abarth-chromium, 2015-01-26 18:53:56]

Committed patchset #1 (id:1) manually as
4393ecb1208f1fbc9aed551b39eaaa2c31dcd18a (presubmit successful).