Default VDA expected coded size to 320x240.
Default VDA expected coded size to 320x240.

Provide a default of 320x240 as the expected coded size when
initializing VDA::Config.  This was the default prior to
https://codereview.chromium.org/1869103002 , but switched to 0x0 by
that change.

Any user of VDA except GpuVideoDecoder will use the default.

BUG=602759

Review URL: https://codereview.chromium.org/1915563002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#389228}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk