Avoid exporting read_buf renaming from zlib

zlib.h includes a macro that renames read_buf->Cr_z_read_buf. Since
read_buf is a common name in other parts of the code, it causes
some random confusion depending on whether zlib.h has been
included or not.

The renaming macro is a side effect of the 0001-simd.patch that
exposes an internal read_buf method to other files in zlib. This
patch renames read_buf as it is exposed so that it has the less
common name deflate_read_buf.

Bug: 799448
Change-Id: Icdc4eba973891dfd28d82017415048eded62d577
Reviewed-on: https://chromium-review.googlesource.com/852257


Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Chris Blume <cblume@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#528512}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 2c709d38a1c6f812da205c03f5448fe4ac5679f3