Fix choppy scrolling when flinging on OS X.

This was caused by a bug in Chrome: wheel events stop firing when source
element is removed from DOM. Workaround: leave element in DOM for awhile.

On the upside, React makes this clean to implement. No need to keep state
to remember to remove the dangling element. It automatically gets removed
when it is no longer the last element to receive the wheel event!

Change-Id: I1b9372bfd1edcfb532f6825d99044a314048c3b4
Merge "Fix choppy scrolling when flinging on OS X."
cmdmain: guard ExtraFlagRegistration with a Once

Change-Id: Id54111dc21eeb61b02721117123b956fcc3b47fd
Limit upload concurrency (number of goroutines) in camput for Android

Sidestep the problem by uploading files first, and buffering
directories to upload them at the end.
Limit upload concurrency to 3+2 (normal+directories) goroutines
on Android, and -1+10 otherwise.

Now the test FAILS, as one of the 3 runs in sequence, one WILL block.
Even with buffering all the directory uploads till the end!

Change-Id: Idac4c94b7d7ec4fc6c6a77fed111e0ece0843545