Add compiler warning flag -Wextra and fix related warnings.

Add compiler warning flag -Wextra and fix related warnings.

Note: some of these warnings are enabled by a combination of -Wunused
(added earlier) and -Wextra.

Cherry-picked from AOM 4790a69faaec8f03d65f64ff070f6ab4307dbb16

Expands use of (void)x; on unused variables. AOM only supports one codec
in codec_factory.h

Does not include changes to HandleDecodeResult. AOM removed
invalid_file_test.cc which does use the video parameter.

Does not enable -Wextra yet. There are more issues to fix.

BUG=webm:1069

Change-Id: I322a1366bd4fd6c0dec9e758c2d5e88e003b1cbf