Add WEBP to the list of Image formats that support Compression.

Note: The integrator of this change to Android internal code-repo will
have to run one extra step 'make update-api' to update 'api/current.txt'
file corresponding to approved API. The AOSP branch didn't have this
file, hence I could not add the same to this change. The updated file
'api/current.txt' has to be submitted along with this change.

Change-Id: I29909e907a2e82d801e16654322190a808c5bda9