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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.

BUG=393802
R=danakj, enne

Review URL: https://codereview.chromium.org/399233004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285114 0039d316-1c4b-4281-b951-d872f2087c98