Writes PRE_OBU_SIZE_BYTES (currently 4) bytes padded unsigned LEB128
encoded integers in OBU size fields when enabled:

$ cmake path/to/aom -DCONFIG_OBU=1 -DCONFIG_OBU_SIZING=1 && cmake --build .

Requires CONFIG_OBU.

BUG=aomedia:1125

Change-Id: I4d184ef0c8587d24e9c8c3e63237ea5003386c6a