Fix a memory leak in skdiff that happens when the

Fix a memory leak in skdiff that happens when the
directories do not contain the same files.

Only create the bitmaps when we need to use them.

Committed: https://code.google.com/p/skia/source/detail?r=6481

[TomH, 2012-11-19 11:04:04]

LGTM.

Food for thought: If the bitmaps are only being used in that one subtree of the
code, and don't need to persist outside it, do they really need to belong to the
DRP?