Fix HttpStreamParser fuzzer.
Fix HttpStreamParser fuzzer.

Reading HTTP headers returns net::OK on some success paths, and bytes
read on others.  The test didn't account for this.  This CL fixes the
fuzzer to account for that.  The weird HttpStreamParser behavior should
also be fixed.

BUG=598037

Review URL: https://codereview.chromium.org/1868003004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386197}

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