Fixed channel switch when user decides to switch to another channel in
he middle of current channel transition.
For example:
1. User is currently on the beta channel and decides to switch to the dev channel.
2. In the middle of the switch the user decides to switch to the stable channel
instead of currently downloading dev.
But update engine moves to UPDATE_STATUS_REPORTING_ERROR_EVENT because
target channel isn't equal to downloading channel, so request to start
downloading of stable channel fails. Almost immediately update engine moves
to UPDATE_STATUS_IDLE, but user needs to reload chrome://help page to initiate
update check. Current changelist handles the issue and implements automatic
update checks when update engine is idle.
BUG=311162
TEST=unit_tests:VersionUpdaterCrosTest.*
Review URL: https://codereview.chromium.org/
578293002
Cr-Commit-Position: refs/heads/master@{#296693}