If an HTTP request includes the header:
X-Gerrit-RunAs: spearce
and is made by a user granted the runAs capability the request
will evaluate as though it was run by user "spearce", and not the
calling user. This roughly matches the behavior of suexec over SSH,
but for HTTP API requests.
The new header takes any specification for an account, accepting
Account.Id, email address, full name, or username.
Change-Id: Ieda546a0db7290e31a8941c1b761c219aa0b63f9
bb027b04d4