Make sure that HttpRequestHeaders contains valid key-value pairs.
Tracked down all uses of SetHeader() and SetHeaderIfMissing() and added
input validation where necessary (using the new IsValidHeader{Name,Value}
methods in http_util, moved from web_request_api_helpers).
After that, I added a DCHECK which serves as documentation to the users
of http_request_headers: The input must not contain illegal characters.
BUG=390458
Review URL: https://codereview.chromium.org/491123004
Cr-Commit-Position: refs/heads/master@{#291761}
11 files changed
