Fix ext-tile without ext-partition.
Fix ext-tile without ext-partition.

Default case (when ext-partition was not configured) was incorrect
in encoder tile size initialization.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1197

Change-Id: Ibe57cb1dc16b9fa300573816fc16d2d2f6849fc6

mbebenita/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,luctrudeau/aom,smarter/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,smarter/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,luctrudeau/aom,GrokImageCompression/aom,mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom