Fix potential XSS in docserver.
Fix potential XSS in docserver.

Fix the bug where this URL will reflect the cl parameter value back into the response:

   https://skia.org/path/to/markdown-file?cl=%27-function(){debugger}()-%22%3E%5c%22%3E%3CscrIpt%3Edebugger%3C/scrIpt%3E%3CaUdio%20src%3dx%20oNerror%3ddebugger%3E%3C%22-%27-function(){debugger}()

BUG=skia:

Review URL: https://codereview.chromium.org/1445223002

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot