API: Recognize an "Api-User-Agent" header

XMLHttpRequest does not allow overriding the "User-Agent" header. To
allow such clients to make effective use of the api-feature-usage log,
let's give them a different header they can use.

Change-Id: Ia1e4f52f35f8d9a8e9f5590a00854d14c11f7cc9
Merge "API: Recognize an "Api-User-Agent" header"