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In https://crrev.com/c/997098, params_ was changed to a local variable,
but it needs to last longer than that since net::WrappedIOBuffer may use
the data after the local variable goes out of scope.

This CL changed it back to be an instance variable.

Bug: 851799
Change-Id: I392f8acaef4c6473d6ea4fbee7209445aa09112e
Reviewed-on: https://chromium-review.googlesource.com/1103676Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568137}