Fix ext-tile encoding when not using compressed header
Fix ext-tile encoding when not using compressed header

The compressed header size is now sometimes not signalled and the
encoder code hadn't been updated to match.

Note: This mistake caused the failing test tracked in bug 917 to fail,
      but fixing it doesn't fix the test: the decoder now staggers on
      a little further before getting out of sync.

BUG=aomedia:917

Change-Id: Ic0de99ede778844c4dbdd34dba3fba9c1fa2d95f

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom