Bug 1258470 - 3. Small optimizations in ThumbnailHelper; r=nalexander

This patch includes a small memory optimization of using ArrayList and
`volatile int` for storing the pending thumbnails list and pending
width, instead of using LinkedList and AtomicInteger, respectively.

The patch also fixes a possible race condition due to calling
processNextThumbnail outside of a lock. Now it must be called inside a
lock and its name is changed to reflect that.