Currently, when scrolling a page, Chrome splits tiles into Now, Soon, and Eventually bins. Eventually-bin tiles are not typically pre-rastered, and if they are using up memory, it's because they were rastered/displayed at an earlier point, and have since moved out of the Soon/Now bins.

These tiles can be dropped with minimal impact on performance (dropping them just leads to a bit more work / power consumption if we scroll back to them).

We should consider dropping these Eventually-bin tiles after a time-out, to prevent memory usage from growing and staying close to the tile-memory-limit on long pages. This should reduce peak memory usage for users with a large number of foreground tabs.

We might consider:
- Dropping eventually tiles if they stay in the eventually bin for >30s.
- Limiting this behavior to windows without focus?