Explicitly coerce PostDispatchAction to uint32_t in DispatchEvent()

Intermingling enums with integer types leads to warnings on some
compilers. Explicitly coercing an enum to an integer before returing
it clears up the problem.

Broader changes (using constants instead of the enum, returning an
enum instead of an integer type) were considered but deemed too
sweeping.

BUG=424334
TEST=ui_unittests, events_unittests
R=sky@chromium.org

Review URL: https://codereview.chromium.org/669193002