Send proper MIME type along with PPAPI test CSS files.

https://codereview.chromium.org/1126133003 changes the behavior of
Blink CSS file support; we need to send the proper MIME type, even in
quirks mode. This CL fixes the tests which broke when that patch
initially landed way back when.

BUG=419383
TBR=bbudge@chromium.org

Review URL: https://codereview.chromium.org/1136493003

Cr-Commit-Position: refs/heads/master@{#329358}
3 files changed