net: Remove FilePath version of URLRequestMockHTTPJob::GetMockUrl().

For historical reasons, URLRequestMockHTTPJob::GetMockUrl() and
GetMockHttpsUrl() take a base::FilePath even though the only thing they
do with it is call path.MaybeAsASCII() on it. All the unittests using them
have to do ugly things with FILE_PATH_LITERAL when calling.

This patch fixes the only file using the FilePath version of GetMockUrl().

BUG=496936
TEST=browser_tests && unit_tests
R=mattm@chromium.org,mmenke@chromium.org
TBR=thestig@chromium.org,avi@chromium.org

Review URL: https://codereview.chromium.org/1391423002

Cr-Commit-Position: refs/heads/master@{#354691}
20 files changed