Cherry-pick: (Reland) cc: Remove recycled tilings when active tree removes them.

Clean cherry-pick of

BUG: 16548433

Original description:

This patch is the second part of ensuring that recycle tree does not
contain any unshared tiles.

This is done by removing recycle tree tilings when active tree removes
those tilings.

This includes a check in RemoveTilings for NULL tilings. When the first
pending tree activates, it swaps with active tree's NULL tilings, which
become the recycle tilings. When removing tilings, we have to ensure
that tilings is not NULL before accessing its members.

Change-Id: I29b3c0514cac1fc8f31251ff00f65a82631eaf65
4 files changed