Changing webmdec to use libwebm for WebM file parsing.

Change-Id: I2a57a7b44dbed05eaa04409e1e75e6fc03b30fbc

Fix return values for webm_read_frame so that we can distinguish between
error and end of stream. 0 - Success, 1 - End of stream, -1 error.

Change-Id: Ic35d0c7d7a166e027711a3d2300ecdda25a5d0cc

Change-Id: I5164df72aff84eca0ace56032c5373f04053c6a5

- Move it to webmdec.c and webmdec.h.
- Also, tidy up obvious style nits in the vicinity of code I was
  already touching.

Change-Id: Ie2898d06e73c1e9030d9c8d465b73ee7edc3c02a