Currently layers reserve textures in the texture manager when their contents are painted and unreserve after the layer has been drawn. Since the compositor can skip layers at draw time (e.g. when the layer is outside the clip rect) some textures are left reserved. Over time they add to the memory (VRAM) bloat.
The debug chromium builds currently assert when a texture is reserved twice (which is an indication that the texture was not unreserved after the previous draw).
An easy repro case for the assert is loading this page:
Created attachment 90577 [details]
Comment on attachment 90577 [details]
Looks good. I'm pretty sure this will conflict with enne's upload patch.
Committed r84582: <http://trac.webkit.org/changeset/84582>