zip: Use SequencedTaskRunnerHandle in ZipReader

This CL replaces all references to ThreadTaskRunnerHandle in ZipReader
with SequencedTaskRunnerHandle.

BUG=752702
TEST=zlib_unittests

Change-Id: I2f6ffc38312e025c7a6f60275fd651cd7d1f062a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569301


Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833655}
GitOrigin-RevId: d9769fd5efc44e4545432354f1f0251891b56c6b