One of our internal Chromecast builds was failing due to undefined
va_list in wasm-result.h. I also searched for other files where va_list
was used without including stdarg.h and added it as necessary (since
include-what-you-use is a thing).

BUG=chromium:706443

Review-Url: https://codereview.chromium.org/2780913002
Cr-Commit-Position: refs/heads/master@{#44588}