Fix memleak in WebPIDelete (change#Id4faef1b).

Corresponding bug for this fix is b/10823924

Happens when decoding is partial (past Partition0), without error and
interrupted by calling WebPIDelete()
WebPIDelete() needs to call VP8ExitCritical() to free in-flight
resources.

The upstream change for WebP has landed WebP git via gerrit patch:
https://gerrit.chromium.org/gerrit/#/c/67199/


Change-Id: Iae0fb5d1bd239f9641503f08ffb79780da7aa42f